The best way to date an individual Gibson amplifier is to look for the EIA source-date codes stamped or printed on potentiometers. This code also appears on speakers and other components.
Bruce Collins of Mission Amps, coordinated a project to build a tweed Deluxe 5E3 style guitar amp. The goal of the project was to build high quality, great-sounding clones of the immensely popular narrow panel Fender tweed Deluxe 5E3 circuit at a significant savings over boutique prices. This model is ideally suited for both the beginner and advanced builder due to its relative ease of construction and killer tone.
He received about 50 responses from people interested in joining the project, many for whom, including myself, this would be their first homebrew amp. Since he is a custom amp builder and repairman with a relationship with parts suppliers, he offered to gather the chassis, transformers, and all other electronic parts in the form of a kit. Project members were on their own as to cabinet and speaker options. He also supplied a high quality schematic, color-coded layout and help via email as needed.
Histories of Tube Companies
The RCA pages have lots of pictures of the factory back in the day
1. insufficient decoupling between stages
2. insufficient capacitance or bad caps in the B+ filters
I'm still having motorboating issues with the Magnavox when I have all 4 6V6 tubes installed. It seems that if I swap the tubes around it sometimes goes away for a while. Sometimes it occurs even with only 2 tubes installed. The 4 new 6V6GT's I got are marked with their test values, so I have been trying to match pairs with close values in each channel. I thought I had it sorted, listened with no problems for an hour or so, then turned off the amp, then turned it back on and it started motorboating immediately.
So I got it on the bench and bypassed the power supply filter caps with new individual capacitors, since several folks on audioasylum.comsaid it was probably the problem. Still motorboats. Argh. Hopefully implementing resistance coupling will help.
"[Tube guys] look at the audio system in a different way. The source, preamp(do we need it ?), and power amp pre-driver should be consider[sic] as one section that should be as neutral and transparent as possible. This section is where low signal is tranferred. Any deiviation or colouration will be greatly magnified by output section. The power amp and speaker combination is considered as another section since there is a lot more complicated interaction between power amp and speaker. This section is where we choose our personal preference."
P.L.C.Lam Consulting Inc.
Tonight I installed a lighted switch into the Magnavox, as well as a fuse holder. It has a 1 amp fuse in it and so far so good, although it''s doing that motorboating thing again unless I pull one of the power tubes. It didn't do it when I had it hooked up as 16 ohms (2 8 ohm speakers in series per channel). Moved the Foreplay and the Mag into my workshop so I can play with it with different speaker configs. I'm not convinced the Magnavox is truly a stereo unit - I saw a post where someone mentioned the they made a version that had one channel for low frequencies and one for high frequencies, like for a TV, in mono. It may explain why it seems muted... but damn if it's mono it sounds gooood! I need to test it with an obviously stereo CD. if it is a mono unit I'll make it into a sweet-sounding guitar amp.
So, I built my Foreplay pre-amp, and it sounded great with the Magnavox, but with the ST-35 it has WAY too much gain, and I can hear a lot of buzz and hum. It has been suggested:
What you will need to do to start with is lower the overall gain of the FP. There are wads of post on this subject in the archives. and VoltSecond's page in 'Loose Parts' has an excellent section on attenuators. The quickest, cheapest, easiest fix is the Simple Shunt.
Once you get the gain where you like it, you may need to tackle some other issues, such as grounding and noise reduction, but geting the "gain chain" right is the logical first step, IMO.
Simplified instructions for Simple Shunt for Simple guys like me:
1. Using a 100K resistor instead of a wire, connect right tab of left L-Pad to left inner contact of selector switch.
2. Connect center tab of left L-Pad to A-2.
3. Connect left tab of L-Pad to terminal 14.
4. Using a wire, connect center tab of left L-pad to Right tab of left L-pad.
The Easy Way to make the Stock Foreplay Volume Control into a Shunt Mode Volume Control:
1. Remove the wire connection from the top (right terminal) of the volume pot to the selector switch at the volume pot end.
2. Insert a resistor between the volume pot top (right terminal) and the loose wire going to the selector switch.
This resistor is typically 47.0K (3.3 dB minimum attenuation) to 200K (9.5 dB minimum attenuation) metal film when using a 100K volume pot.
3. Run a short wire from the wiper (middle terminal) to the top (right terminal) of the volume pot.
Using this method (Shunt Mode Easy Way) will reduce the any noise you get when you adjust the volume when compared to (Shunt Mode Pot Typical). I have been told it sounds better this way too. I didn't notice too much of a change with the stock Foreplay pots, but I didn't have a hot rodded Paramour to drive my speakers at that time either.
From VoltSecond's web page
Here's some Thordarson data that's free: Thordarson Manuals (top of page)
I'm reading this book right now. It's pretty good at explaining stuff, but it is very dense, and the technical level changes a lot. I think he was trying to save space for all the schematics people have been bugging him for for years.
4 Hudson Street
David Greenberg email@example.com
781 322 5560
FAX 781 322 0601
This place is right near me, and the website looked promising, so I sent them an e-mail about getting some training.
The Ultimate Tone offers the technician or hobbyist a wide information base for properly implementing guitar amplifier modifications. Sonic perceptions and stage setups are analyzed to demonstrate how optimal sound can be obtained from existing equipment. A survey of amplifier brands follows, with an eye toward ease of modification and service. Basic vacuum tube operation is clearly presented, as are power supply principles. Preamp circuits and master volumes are explored with a discussion of guitar sustain and interactions, for clean and distorted tones. Power amplifiers are investigated, and power tubes are explored regarding choice of tubes, relative reliabilities of tube types, the relevance of tube matching, and the sonic impact of each tube type. Methods of improving reliability are presented. Tube and solid-state effects loops and reverb loops are investigated, with relative performance issues highlighted.
An extensive discussion of switching methods is presented, a subject which is rarely explored with regard to guitar systems. Channel, reverb, effects, lead boost, mute and speaker switching are explored. Foot-switch controllers with and without LEDS are detailed, with an eye to multiple controllers for larger stages.
Tube data is provided for the seventeen most common types used in guitar amplifiers, with pin-outs for nine less common numbers.
Replacement transformer data is provided for the two Hammond Mfg. model lines which cover most guitar applications.
The intended reader must be able to work from a schematic.
It could be a coupling cap----though I haven't seen one that caused hum
to be injected into a system. If the bias is holding a steady settling,
then the diode and caps for the bias power supply are probably okay.
But, if the bias voltage (usually around negative 30 volts, for
6BQ5/EL84 tubes) is NOT steady---go after this part of the circuit.
I'd change out all electrolytic caps, use carbon comp replacement
resistors of the same value, a new diode (WATCH the polarity!!!----the
bias supply has the caps and diode facing the opposite direction, as the
main power supply). Eg: diode's "band" or positive end FACES the PT
and the 'lytics have the (+) ties to ground. Any Zener diodes in this
part of the circuitry should be replaced with like value new Zener.
Bad bias supplies not only knock out power tubes and associated
components, but they also can inject hum and false overtones into the
Some Dynaco amps put an 0.02 mfd cap (usually disc type) from the center
tap of the (6.3 VAC) heaters to ground. Test the two legs of the
heater. Use the VAC setting on your DVM---red lead to each 6.3 volt tap
the black to the point where that cap is grounded. Now, if the voltage
are more than 10-20% off each other (eg: one is +3.4 VAC and the other
is -3.0 VAC) a healthy 60Hz hum will be injected into the system (due to
the 0.4 volt difference in peak-to-trough heights, in this example).
60Hz hum is quite different than 120 Hz noise (which is usually injected
by a bad PS or bias supply). Kinda loud ruckus, which makes you think
the woofer is going to blow.
Make sure that cap is replaced and both legs give you close to the same
voltages. If not, I'd put in a "hum balance" pot which ties the two legs
together through a 100 ohm pot. Each leg to each end (to furthest tags)
on the pot. Center post or wiper to ground. What you are doing is
making a voltage divider which shunts the excess voltage of one leg to
ground----so that both legs make the same ABSOLUTE VAC. Twisting the
leads from PT to each tube should help cancel out EMF induced hum. A
small 1/2 watt linear pot is all you need.
Well, it's a thought. A lot of hum of a powerful nature usually can be
traced back to the heater taps (remember these boys pull amp levels of
VAC to the tubes) and/or bad can or mis-wiring around the can and/or PT.
Good luck. Keep me informed.
Didn't want to post this, but did you modify the PT or OPT when you put in the
new electrolytic caps? When the amp is turned off the magnetic field around the
transformers collapse and convert to a brief electrical surge. The bigger the
iron, the bigger the surge. That after market filter board may not have taken
EMF collapse of a bigger tranny into consideration.
The PS filter caps and chokes' values are designed to absorbed part of this
flyback. I know that guitar amp DYI'ers get into hellacious trouble when they
start fooling around with PS caps & trannies. Sometimes, they get a loud whack
(loud enough to ruin a speaker), when they turn their DYI amps off. Reason,
seems to do with time constants & letting the components get TOO FAR from
nominal values. O'Connor, in his TUT1 and TUT4 has a great dissertation about
this problem and how to solve it----the cheapo way and the correct way.
Assorted Isolation transformers including a Thorardson one for $35
"Thanks for the in-depth reply"
Glad I could help.
"On second check I can barely see the number 35, possibly followed by a W, on the mystery tube."
Yep, a 35W4 7 pin rectifier. So look for a big resistor like I mentioned. Be sure it's in good shape.
"Someone started to make a tube layout diagram on the inside of the cabinet, but the only value filled in is the 12AU7, and of course I pulled both other tubes without noting their original locations."
The 12AU7 number is probably 12AU6. All the sockets are 7 pin, right?
"How would I figure out which tube goes in which socket?"
Okay, if you know where the 12AU6 came from, then the 35W4 goes to a socket that connects to one side of the line cord - probably through that big resistor I've been talking about. If you find that, then the 50C5 goes in the only remaining socket.
"The sockets are on either side of a smallish paper-sided transformer (is this the "choke?")."
No, that's part of the filament supply for the 12AU6 IIRC.
"There is also a bigger, all-metal transformer, "Tranco 44 36 NOV 1963", and a 3-section filter cap can. Is this a power trans?
No, that's probably the output transformer. And the triple cap is a power supply filter cap.
If it'll help, here a link to a site that has a schematic that's probably pretty close to your amp's:
Scroll down and click on "Bargain Bin Amps", then click on the Harmony H-400 amp. That schematic is pretty typical of the old transformerless 3 tube amps.
Be safe, but have fun!
Posted by Jim McShane
Re: old Lindell guitar amp - no info on web - tube id - ?'s
Let me see if I can help...
"The tubes light up but no sound, not even a buzz. One of the wax and paper
capacitors is blown apart, can't read the values, but there is another capacitor
that looks the same and it is marked ".01mfd +-20% 600 W.V.D.C. Dumont 157." It
doesn't have a line around one end or a + marking."
"- is this an electrolytic?"
No, it's a coupling cap. Replace both of them, if you haven't got proper caps
(or tubes) drop me a note privately.
"- does this cap being blown up point to a likely suspect?"
Hard to say, it's so old it may have just come undone on its own. The tubes are
definitely suspect, as are the filter caps.
"The rectifier tube is a 12AU6, then there are two other tubes, one is marked
50C5, the other is indecipherable, but looks to be similar in construction,
except it has a clear top.
Actually the 12AU6 is not the rectifier, it's a small pentode that's used as a
voltage amp. The 50C5 is your output tube, and the mystery tube is likely to be
the rectifier. It might be a 50DC4, but...
The amp probably does not have a power transformer, and the tube heaters are run
right off the AC line. So if you add 50 volts for the 50C5 and 12 volts for the
12AU6, then the rectifier needs to drop about 50 volts, that's why I mention the
50DC4 (50+50+12=112 volts-close enough to 115). Some units also used a resistor
in series with the heaters, and they often used a 35W4 rectifier. I'd bet you
find a large resistor of 100 ohms or so in there, and that rectifier is a 35W4.
The 35W4 was VERY common.
The B+ is developed with a 1/2 wave rectifier runnning straight off wall AC too.
This is a DANGER! Don't run this amp or test this amp while it's plugged in
unless you know EXACTLY how to handle it. The chassis may be "hot", so a nasty
shock would occur if you touch the chassis and an earth ground at the same time!
I STRONGLY recommend you use an isolation transformer to work on or run this
Replace all the caps you can find, replace the tubes (they're cheap!), clean up
the controls and tube sockets, jacks, etc., GET AN ISOLATION TRANNY, and you'll
probably be good to go. You might find some resistors that act up or are
cracked/damaged, but the caps and tubes are a must, as is the ISO tranny. Be
sure the line cord insulation is 100% too!
Pics of the Lindell:
Now I'm digging into trying to fix this old Lindell guitar amp. I haven't been able to find a schematic for it, but it looks pretty basic. Here're some articles and sites I've come across...
So How The Heck Do Tubes Work, Anyway?
Hoffman Amplifiers - schematics and articles
http://www.schematicheaven.com/ "Where all good designs await resurrection..."
6CA7 = EL34