Last night I fixed my ST-35! I scavenged the power transformer out of the SCA-35, and used the new quad can capacitor I had made for me - though I guess I could have just used the one from the SCA. I soldered everything back together, also stole a couple of resistors from the SCA, and fired it up gradually with the Variac, using a low value fuse just to be safe (1.5A instead of 2A)... Now it's hooked into the main system and sounds amazing - MUCH nicer than the SCA-35 did. I remembered really enjoying this amp and now I realize why - the SCA-35 was kind of muffled-sounding, the ST-35 sounds much more crisp and has better highs. And I fixed it myself! So psyched to have actually succeeded at one of these projects, I'm now feeling more confident to plunge into something more challenging - such as fixing this old Challenger PA amp (bad hum) and then converting it into a guitar amp! The adventure continues...


I've gotten some feedback on this amp from Tube Asylum

screw-on microphone connectors for Astatic JT30 and other high impedance mics

Modifying Guitar Amps for Safety


Tube Amp Debugging Page!

Sean wants me to convert his Fender Super Six amp back to this mod Tim had done to it many years ago, then Sean had it undone...

Hi Josh - Re amp
project, Sean wants to swap the output tubes in his (my old) amp back to EL 34s
(the way I had it) from its current 6l6. Found a couple of places on the web
with appropriate documentation and it looks like a pretty simple deal. Think we
can knock it off in an afternoon as we dont have to switch sockets. Also might
want to take a look at the input section as the schematic shows it has swapped
in lower powered less bright tubes (cant remember the # maybe the 12AT7 ) for
the standard 12AX7 and I dont remember what I tubed it with, or for that matter,
what Sean has put in it over the years. Also should give the pots a cleaning and
replace the knobs, about half of which are broken.

I think this is the kit for it: http://www.torresengineering.com/elconkit.html


Got email from Triode Electronics, they had some PA-774's made for them and are taking delivery Friday. $69.


I love this article. In fact I think I've linked to it before.

Still searching for a power transformer (PA-774) for my Dynaco ST-35. They're turning out to be rare and expensive beasts. I;ve heard you can wind your own transformers - too esoteric?

Here's a nice Tube FAQ that has a detailed section on transformers as well as other good tube info:

Designing and hand winding transformers is not terribly difficult, but it does require information and skills that are relatively hard to find. You are unlikely to save a whole lot of money unless used or broken parts are cheaply available to you. You may want to do this if you feel that you were selected by some deity to take this on as a life work. First, take a transformer apart. A burned out tube-type power transformer will do. Do this carefully and slowly, imagining how you would have put it together in the first place to get it the way it was. This is an excellent introduction to the manual skills and materials needed to successfully produce one on your own.

Also, I recently took shipment on this wild amp I found on ebay. It's much bigger and heavier than it looked in the picture, and has cleaned-up gorgeously. I'm afraid to power it up as the underside is quite corroded. Gotta fix that digital camrea so I can post pics...


Here's a PA-774 for 90 Euros

About output transformers

These guys have the transformer I need for the ST-35, the PA-774 (Dynaco part #464774), for $130 Euros... shoulda bought the one off ebay for $25.


Wiring up a 4-speaker console for stereo

Impedance matching

Matching speakers to amplifiers

Matching Microphone to Input

Impedance: matching your speakers to your amplifier

Cool stuff for sale

What the heck is "Impedance" anyway????


Large assortment of Nixie tubes available here


Valve Magazine is a really cool online mag for us DIY'ers. The current issue has an article on how to use some of the tubes in my collection to tube rectify a Foreplay kit! Also, severla other articles on using less-common tubes.


In Reply to: bought a big box of tubes - what are they? posted by total newbie on March 13, 2003 at 10:41:33:

12DW7 - dual triode, half a 12AX7, half a 12AU7
12AU7 - dual triode, mu=20
12AT7 - dual triode, mu=60
6CG7, 12BH7 - dual triode, good driver tubes
6GU7 - like 6CG7, but better
6GK6 - great output tube, same as 6BQ5 with different pin-out
6AF4, 6ES5, 6ER5, 6HQ5 - triodes
6BY5 - nice rectifier tube with isolated 6V filament
You could use two 6GK6s plus the 12AT7 (or a pair of 6AF4s) to make a nice little single-ended amp

- Tom Bavis


The Audiophool Page - Tom Bavis' site

All those tubes I just bought...

Tube Database

Dynaco SCA-35 - integrated amplifier


ST-35 for sale: http://www.antiqueradio1.com/pastclass2/CAForSale2.html

Blonder Tongue Audio Baton technical manual for sale


I find general purpose irradiated vinyl insulated,600V,105 deg C rated hookup wire from brands like Alpha, Carol Belden etc. to be fine performers. You have a wide range of gauges in both solid and stranded (all these brands are tinned Cu) and most colors. The ir vinyl stands up to the rigors of normal soldering w/o melting and looking bad. I tend to use 20 or 22 ga solid for low current apps and 18 for higher current apps like heaters. The 16ga is a little unwieldy even in stranded form so I don't use it much. Digikey carries Alpha and General Cable brands although only in 100ft spools. Actually, 100ft isn't as much as you might think.

Some like teflon insulated wire but I feel that it's overkill for most apps. It's also expensive and gauge and color selection is more limited. Digikey also carries the Alpha brand of this but with the 100ft minimum. Many DYI suppliers also carry tfe wire in much smaller minimums.
- Steve Oda


This guy has a pair for sale for a lot of money:


I want a pair of these:

Why? Pure, unadulterated-by-logic lust! it's a single channel tube eq that uses 12ax7 tubes.

The first commercially available (for the consumer market anyway) tube equalizer in the mid 50's. It was called the Blonder Tongue "Audio Baton", used 12AX7 tubes throughout and had 9 frequency bands. (they had a pretty nifty way of graphically representing the relative amount of gain/cut... I can give more detail if anyone gives a hoot) The designer's name is Ben Tongue (see his website below). He also holds the patent. The schematic is here: http://www.triodeel.com/images/b9b.gif
There was a model B9 and a B9B. The major difference being that the B9B had a slightly increased B+ voltage and the upper frequency band characteristics were changed a bit.
- Gandalfcrow (AA)

This guy replaced all the caps and made his work perfect: http://www.endino.com/photo9.html

It was designed by this guy named Ben H. Tongue

Here's an interview with Mr. Tongue

Awesome page of old tube ads!

The ST35 does tend to run fairly warm, but several things are worth checking as they will contribute to increased heat. Do change the coupling caps (.1uf/400v), they often get leaky. While you're at it change C3 (.22uf/200v) Check the filter cap and replace as necessary. If you can, measure leakage under working voltages, or measure ESR.
C1 (.1uf/16v ceramic) cap does horrid things sonically, replace it with most any poly cap for a big improvement.

A small muffin fan (I like a 220v fan on 110v - runs sloooow) exausting up on top of the case will keep the temp way down.

As you've guessed, the bias is not adjustable - the ST35 uses cathode/self bias. I've tried individual bias resistors in an ST35. Although it sounds like a good idea, it sounded really poor IMHO. I'd stick with the original design. The EI 6BQ5's sound pretty darn nice and are quite inexpensive.

- Gary Kaufman

More ST-35 info from Tubes Asylum:

The Dynaco ST-35 is "Cathode Self Biased" and has a common 90 Ohm resistor attached to all 4 power tube cathodes to Ground that determines the current flow through the power tubes at idle. There is no adjustment. This requires use of a fairly well matched quad of power tubes.
This 90 Ohm resistor causes the power tubes to idle a little "hot" in my view, so in my ST-35 I changed this resistor value to a 120 Ohm (about 15 Watt) resistor.

Also, the tall silver Multi Cap can has 4 Capacitors in it. They are 3 filter caps for the power supply that may or may not need replacing. The forth Capacitor in the Can is the Cathode Bypass Cap, and I would replace it with a better quality cap, but one that would fit in the very small space (barely 1/2 inch) available on the underside of the top chassis

Also, you might wish to upgrade the coupling caps on the two PCB's with a bit better quality. I found that Solens fit the PCB space available nicely.

With just a few upgraded capacitors, and a new set of NOS tubes, the ST-35 will perform at a very high level, as it is one of the very best Push Pull designs ever.

- Ivan303



Get a stable 10amp Variac. After soldering in your new cap AND double
and triple check your connections and polarity!

Replace any indirect rectifier (like a 5AR4/GZ34) with a direct
rectifier (like a 5Y3GT). Plug all tubes in & have speakers connected.
Volume at zero. No inputs.

Start at 60 volts VAC, for one-two hours. Check for anything unusual
(smoke, smells, etc). If all okay after 2 hours, ramp the Variac up to
80 volts (another 1-2 hours). Continue this trend until you reach 118

I find slowly ramped-up or formed electrolytic caps last a lot longer
than ones which aren't.

steve oda

Capacitor can rebuilding, now with rapid return of your rebuilt can. Any twist-lock can rebuilt for $30, up to four section. Maximum 450 volt at that price. Nut mounted cans $20 single section, for multi-section add $2 per section on nut mounted cans only. Shipping add $4 per order for Priority & insured shipping via PO. Rebuilt cans returned only after receipt of check, money order, or credit card info. Our guarantee on all rebuilt cans, 1 year. We will test any can for leakage and capacitance, at correct voltage, for $2. Frontier Capacitor, PO Box 218, Lehr, ND 58460 or 403 S. McIntosh, UPS. Toll free (877) 372-2341. Ph.: (701) 378-2341. Fax: (701) 378-2551, voice mail recording anytime


Strategies to Repair or Replace Old Electrolytics

Strategies to Repair or Replace Old Electrolytic Capacitors

Found a correct replacement quad cap for ST-35's!! (tube asylum thread)



ST-70 troubleshooting guide - useful, but I'd really like an ST-35-specific version...

Here are the plans for the Bushorn Mk2's I want to build for my RS 1197's. The dimensionas are provided in cm, so I converted them to inches and fractions. Yawn. "Math is hard." Here are the non-converted dimensions:

A 13.5 x 99.0 cm - 16mm
B 13.5 x 33.4 cm - 13mm
C 13.5 x 59.5 cm - 13mm
D 13.5 x 13.4 cm - 13mm
E 13.5 x 41.0 cm - 13mm
F 13.5 x 29.2 cm - 13mm
G 13.5 x 43.3 cm - 13mm
H 13.5 x 44.0 cm - 13mm
I 13.5 x 33.4 cm - 13mm
J 13.5 x 40.0 cm - 13mm
K 13.5 x 25.0 cm - 13mm
L, M 35.0 x 99 cm - 13mm

Converted to inches (cm / 2.54, then decimals * 16, rounded up to the nearest 16th):

A 5 5/16" x 38 15/16" x 5/8"
B 5 5/16" x 13 1/8" x 1/2"
C 5 5/16" x 23 7/16" x 1/2"
D 5 5/16" x 5 1/4" x 1/2"
E 5 5/16" x 16 1/8" x 1/2"
F 5 5/16" x 11 1/2" x 1/2"
G 5 5/16" x 17 1/16" x 1/2"
H 5 5/16" x 17 5/16" x 1/2"
I 5 5/16" x 13 1/8" x 1/2"
J 5 5/16" x 15 3/4" x 1/2"
K 5 5/16" x 9 13/16" x 1/2"
L, M 13 3/4" x 39" x 1/2"

Home Depot, here I come!

The plans:



Capacitors tutorial

cool info on crossovers


Arglo Electric supply

Gettens Electric Supply

Allite Electric Supply

A.D.L.Wholesale Electrical


CED Lowell Electrical supply

Electronic Hook-Up

Needham Heights
Wellesley Electrical supply

North Andover
Design Lighting & Elec. Supply

Maine electrical Supply

All electric Supply

Boston ElectricSupply

Roberts Electrical Supply

South Dennis
Cape Electric Supply

West Springfield
LJ Macken Electronics

Abbott Electronics

Play Things of the Past

How to Ship Antique Radios Safely


Replacing Capacitors

Replacing Capacitors in Old Radios

...includes links to pages about putting new capacitors into the old can.

Choosing Values of Large Electrolytic Capacitors
For electrolytic capacitors, the same rules apply, except that you can safely use a capacitance value that is considerably higher. In general, you can go as much as 100% higher than the original capacitance value.

For example, when replacing a 10-mfd electrolytic capacitor in the radio's power supply, it is OK to use a 20-mfd or 22-mfd replacement. Likewise, you could replace a 20 with a 33. The higher capacitance may do a slightly better job of removing 60-cycle AC line "hum" from the audio output of the radio. It is not unsafe to go even higher, but you generally won't notice any improvement and the higher-value capacitors (33 mfd, 47 mfd, and so on) are significantly more expensive. Don't waste money on a 100-mfd electrolytic if your radio sounds great with a 20-mfd unit!

Substituting for Unavailable Values
In a pinch, you can combine two capacitors to create one with the desired value. The rule to remember is that when two capacitors are wired in parallel, their values are added.

For example, say that you need a .04-mfd capacitor, but all you have on hand are .02-mfd units. Wire two .02s in parallel, and�voila!�you now have a .04-mfd capacitor. Likewise, wiring two 10-mfd capacitors in parallel creates a single capacitor of 20 mfd. Observe polarity when combining electrolytics (wire both positive ends together and both negative ends together). Both capacitors should have a voltage rating equal or higher than the original.

Wiring Electrolytics in Series

These guys (www.vibroworld.com) have a quad can (made for Gibson GA-20 amps) with the needed specs, but it is .75" too tall to fit under the ST-35 cage. They make custom cans, so I've requested information about a shorter version of the same thing, plus the possibility of getting some "enhanced" versions made with 600v instead of 475, for longer life. Maybe if I can get a quantity made at a good price I could sell these, since they're so hard to come by. Or I may have just given away the store by requesting it from these guys. Oh, well.

Tubes Vs. Transistors
Is There An Audible Difference?
Russell O. Hamm

Parts Connection has become Parts Connexion

EXPERIMENTS IN ELECTROCUTION Good DIY Audio page - well-written and funny

Effectronics DIY Effects site - good theory and a couple good beginner projects w/ detailed parts lists

Electra-Print Audio

"The finest in handwound transformers and custom tube audio amplifiers"

Antique Radio classifieds, etc

Nice little treatise on tube amps

The Idiot's Guide to Tube Testers

Bottom line? I don't need one.


I was unsuccessful in rewiring my ST-35. I tested the quad and it seemed like it was working okay (I was wrong), so I undid the custom wiring done by the previous owner (who had wired a 60uf cap in to replace a failing cap in the quad) and re-wired it to match the original schematic. It's still blowing fuses, so the quad is most likely toast. I haven't been able to find a suitable replacement that has the four correct values (60uf/450v, 40uf/450v, 20uf/450v, 100uf/25v). I found a few quads that are close, but they either don't fit physically, or I have to add so much stuff to make them work that I might as well just use all separate caps. Also, if I go that route I can custom select caps to improve the bass, apparently.

So I've ordered a set of these boards from Jim Sullivan:

Replacement circuit boards for Dynaco ST-35

Converting old mono PA and Hifi amps to guitar use


Small Bear Electronics - stomp box kits and more!

I've ordered these 2 kits:

(click image for details)


(click image for details)

They seem to be very straightforward beginner-type projects and the result will be cool stomp boxes that I can use and show off to my friends.

Ready-to-build PCB's for effects: http://www.tonepad.com/

The PLACE for guitar effects and amp stuff: http://www.geofex.com/

Firebottle Recommended reading re: tube amp repair

First-time effects builder page

Guitar Effects DIY links!

Clone your own effects pedals


Capacitor Testing, Safe Discharging and Other Related Information

Antique Electronic Supply

Hey, more useful advice!

Re: ST-35 - blowing fuses - help!
I would bet the Quad cap is going bad and a previous owner bypassed the first
bad section and no another is going south. Just order a new 4 section can from
www.tubesandmore.com and rewire it per the schematic. They have 2 that should
fit the bill a original with twist locks and a JJ telsa that clamp mounts which
is about a third the cost but doesn;t look factory original.

Good Luck

FaradNet The Capacitor Resource

Capacitor sites database

Ramblings About Audio - various interesting papers from the Decware site

Undestand the Long debate Tube vs SS

Walter G. Jung and Richard Marsh



Stock up on the following: resistors (good mix of 1/4W, 1/2W and 2W types as well as some 5W/10W cement power resistors), capacitors (different values and atleast 350V ratings), switches, grommets, input/output jacks, wire (different gauges and colors), knobs, hardware (machine screws, toothed washers and bolts) and terminal strips. Try to land power transformers, empty chassis, common tubes (6SN7, 12AX7/12AU7, 5AR4, etc), random output transformers, etc. as well. Remember - think like a pack-rat, act like a pack-rat.... be a pack-rat.

Quad ESL speakers - look into this later

Torres Engineering - interesting guitar amp info and kits

Capacitor Tips for Tube Radios

Hi Voltage Caps for vintage radios

Searching for a quad:

I'm looking for a replacement quad capacitor for my Dynaco ST-35.

It was Dynaco part # 297227, 3" tall, 1.375" dia


60mfd /450 V
40mfd /450 V
20mfd /450 V
100mfd /25 V


Is this what I need?

(Response from Audio Asylum posting)

Yep that is the 60mF section that is left unconnected which was the first
section after the diodes. If you look at the side of your cap it will show the
values and the symbol callout that correspond to the terminals on the bottom.
It was probably bad (the voltage is highest closest to the diodes) and left
unconnect just to get the amp working at some point in it's life. The voltage
would probably be close even with this section unconnected since your power
supply resistors still appear to be in the circuit. however the AC ripple would
not be totally filtered out of the B+ voltage going to the circuit, and the
noise and hum to the rest of the circuit will be higher.

The quad cap you found could work but it will fall short of what was originally
in the amp. The original quad cap in that amp is a four section. The first
three sections being 60mF, 40mF, and 20mF all at 450Vdc for the high voltage
filtering. The last 100mf at 25Vdc is for the bias supply.

You can check out Antique electronics supply I think they have some quad caps
that will take care of the high voltage but you will have to install a modern
100-220 mF 25Vdc minimum for the bias under the chassis (new caps of this value
are small and should fit no problem). Also this quad cap is taller than the
stock one and the cage will not fit with it installed.

Or you can use a JJ or LCR 50/50mF at 500vdc clamp mount for the first two
sections (you'll have to drill holes to mount the clamp however) and hide a
20-47mf at 450vdc under the chassis for the third cap (I don't know of one but
there may be one by Panasonic or Xicon or someone else that will fit). And
you'll still have to use a modern small cap for the bias here as well. Check
out Mouser or DigiKey for the modern type stuff.

I have not found a totally stock type solution for this amp and If you find one
please post it. My Stereo 35 has the original quad cap still in it and I know
one day I will have to replace it.

Triode Electronics - parts for Dynacos

Rebuilding an ST-70

Curcio sells an electrolytic quad capactitor replacement board for $75

Here's the site that board design came from. This guy does a lot of cool stuff: http://www.quadesl.com/

Here's a board that replaces the quad cap in a Dynaco PAS amp - maybe it will work for my ST-35?

Build a Nixie Tube digital clock!

Signature Sound - Liverpool NY - used gear, etc

ST-35 Manual for $20+ ship

John Sullivan's ST-35 boards Page

He sells replacement output boards and a filter capacitor board that replaces the stock 4-way filter capacitor (quad)

Dynaco ST-70 Troubleshooting - might help with my ST-35



Library Books

I just went to the library and got out four books:

Understanding Electricity and Electronics by G. Randy Slone
A Beginner's Guide to Making Electronic Gadgets by R.H. Warring
Troubleshooting and Repairing Audio Equipment by Homer L. Davidson
How to Read Schematics by Donald E. Herrington


Converting to Cathode Bias

Converting to Cathode Bias


Learn Electronics - downloads




0nline electronics tutorial


Last night my Dynaco ST-35, which had been sounding so amazing, suddenly faded and died. When I went over to the amp I heard hissing and bubbling sounds. I unplugged it and since it was late i went to bed. I woke up this am and saw the fuse was blown, tried replacing it, it blows when the power comes on. Opening it up didn't reveal any obvious cooked resistors, capacitors, or wiring. Some of the wiring has been re-done, sort of sloppily. There are a few wires with slightly melted insulation, a few ugly solder joints, the circuit boards are a little heat-stressed, but nothing jumps out as fatal.

Dynaco Troubleshooting

Audio Basics Newletter

Transistors vs. Tubes?

W. Marshall Leach, Jr., Professor

- tube amp designs and detailed plans from an all-around wicked smart guy -

Recommended tube tester repair-woman - good info on Hickok tube testers

A recommended list of Hickok tube testers, and Ebay buying tips.

Some other guy's tube info page


The Kappellmeister loudspeaker


Audiophile record label: Chesky

"The Ultimate Demonstration Disc" Catalog No. UD095 Price: $15.98

The Berning Zero Hysteresis ZH 270 OTL Stereo Amplifier by Martin Appel

OTL = Output Transformer-Less Amplifier

An amp design that uses a multitude of power tubes connected in parallel to generate enough output current to drive loudspeakers without output transformers.


Types of drivers

Great lesson on enclosures and speakers from Fostex site

Fostex recommended enclosures

Recommended enclosure for FE103

Fostex user forum

Karlson Speaker Homepage

- John E. Karlson

Interestingstuff about Bass Cabinets

Plans for a TQWP using a FE103 (or 1197)

TL Cabinet design by Martin J. King

A folded pipe design for 1197's with plans in inches

Buschhorn Mk2 pdf (metric)

CAT5 Network cable speaker wires project

TQWP plans - detailed - for 1197's or FE103's (requires 2 per side)

Solitary Man - Neil Diamond

Acronym check:

ML = Mass Loaded
TQWT = Tapered Quarter Wavelength Tube
TQWP = TQW Pipe?
TL = Transmission Line
FTL = Focal Transmission Line (or Faster Than Light)
TS = Thiele/Small (parameters)
SPL = Sound Pressure Level

Martin King's Quarter Wavelength (TQWT) website with Matchcad Worksheets!

Bob Brine's pipe site

Testimonial to single driver pipes w/ 1354's

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