It's amazing how long research takes

[Bob Womack]

I got a request for info about a replacement reverb tank for a classic amp. A guy had read my page about the 1970s Traynor YRM-1 Reverb Master amp. The reverb on his amp had gone down. For some that can only mean that the spring tank died but the first things to go are the connections to the tank and the tubes. I wrote the boilerplate instructions to check connections and tubes and then started on finding the tank.

This is where the task gets interesting. Looking at the amp's schematic we see that an "Accutronics 4L" tank is spec'd. Those who know are aware that there is no such official designation. About all we can surmise is that it is a Type 4 long. Listening to the reverb we can tell that the reverb rings out for a medium or long time as springs go. The input and output impedance across the tank coils is listed as 150 ohms and the tank input is grounded but the output is not on the schematic. The tank is mounted open-side down on the bottom of the amp head enclosure. In this design the reverb driver tube is a 6BQ5(EL84) and the recovery tube is a 12AX7. Use of an EL84 is unique in this design (most use a 12AX7) and makes it a bit different from other designs.

The type codes for Accutronics tanks follows the scheme XXXXXXX, where the first digit is the type, the second is the input impedance, the third is the output impedance, the fourth is the reverb decay time, the fifth is the connection type (grounded or not), the sixth is always "1" and specifies whether there is a locking mechanism, and the seventh is the mounting plane.

A matrix of the info can be found here: AMPLIFIED PARTS

Using the info there I can assemble the following code: 4XX2B1B where "x" is info we don't have. That's type 4, x,x, medium decay, input grounded, no lock, and horizontal open-side down. We can go with a "2" or "3" in the fourth position. The problem here is that the impedance on the schematic is the impedance across the coils and no circuit impedance is offered. I tried a million and one ways to compare that info to online info about various other models and couldn't reliably dial it in. Eventually after two hours of looking I gave him the 4XX2B1B code and told him to consult the US dealer above and Accutronics in Korea to figure out the proper impedance for the inputs and outputs.

All of that took two hours. Amazing.

Bob

[MBDiagMan]

Thanks for posting Bob! I wish you had done this several weeks ago. I ordered a reverb unit for my German Schaller amp week before last. I had to research physical sizes and input/output impedance. I got the reverb a few days ago and I hope to get it in and do some other work the week after thanksgiving. I am anxious to see if I can save my old relic. I have had it for 48 years so I hate to let it go now.

[Steve DeRosa]

Originally Posted by Bob Womack

...In this design the reverb driver tube is a 6CA7(EL84) and the recovery tube is a 12AX7. Use of an EL84 is unique in this design (most use a 12AX7) and makes it a bit different from other designs...

I think you meant EL34, Bob; if you recall the first-series Music Man amps used 6CA7's as power tubes and, speaking as the former owner of a 410-65, a pair of those in a 4x10" cab could eat a '59 Bassman/Super Reverb for lunch and hardly break a sweat. Kinda potent for a reverb driver tube and, given that most built-in tube reverbs I'm familiar with use a 12A-7 tube of some kind, I'm curious about the design of a circuit that would even use the relatively low-powered EL84 as a driver...

[Bob Womack]

Originally Posted by Steve DeRosa

I think you meant EL34, Bob; if you recall the first-series Music Man amps used 6CA7's as power tubes and, speaking as the former owner of a 410-65, a pair of those in a 4x10" cab could eat a '59 Bassman/Super Reverb for lunch and hardly break a sweat. Kinda potent for a reverb driver tube and, given that most built-in tube reverbs I'm familiar with use a 12A-7 tube of some kind, I'm curious about the design of a circuit that would even use the relatively low-powered EL84 as a driver...

It surprised me as well when I first saw it but it was an EL84. Now, I got the U.S. designation above wrong - it was 6BQ5/EL-84, so I will edit that. Thanks for pointing that out. But they did use a power tube to driver the reverb. The actual power tubes on the amp are EL34s, by the way.

SCHEMATIC - Look at the center top.

Bob

[MBDiagMan]

Man, you guys have me thinking I am young again. In 1968 we learned tube electronics only in preparation for learning the Nike Hercules missile system. I didn’t get into solid state until college after the Army.

Tubes are ancient history in most worlds, but in audio they do away with the ragged signal near 0 Volts that make audio sound funny. It’s great knowing that old tech is best tech for some things.

[clintj]

Originally Posted by Steve DeRosa

I think you meant EL34, Bob; if you recall the first-series Music Man amps used 6CA7's as power tubes and, speaking as the former owner of a 410-65, a pair of those in a 4x10" cab could eat a '59 Bassman/Super Reverb for lunch and hardly break a sweat. Kinda potent for a reverb driver tube and, given that most built-in tube reverbs I'm familiar with use a 12A-7 tube of some kind, I'm curious about the design of a circuit that would even use the relatively low-powered EL84 as a driver...

An EL84 as a reverb driver is not too outlandish. Definitely an odd choice, though. The standalone reverb unit Fender made (the 6G15) used a 6K6 power tube to drive the reverb tank, and those will make 3 to 5W in single ended class A depending on the available voltage.

The paralleled 12AT7 driver in the later blackface amps should make a couple of watts max, given the max dissipation is 5W total and you can get about 1/3 of max as usable audio power in class A.

[Steve DeRosa]

Originally Posted by MBDiagMan

...In 1968 we learned tube electronics only in preparation for learning the Nike Hercules missile system...

Um, you didn't smuggle out a batch of '50s/60s JAN tubes by any chance, did you - the hardcore Fender guys would sell their 'nads for a complete set of those...

[Bob Womack]

Originally Posted by Steve DeRosa

Um, you didn't smuggle out a batch of '50s/60s JAN tubes by any chance, did you - the hardcore Fender guys would sell their 'nads for a complete set of those...

Um, I got my hands on a few JAN tubes from a friend:

Tung Sol USAF JAN 12AX7WA
Sylvania JAN CHS 5y3GT
RCA USN-5y3WGTB

They sound great in my Fender types.

Bob

[MBDiagMan]

I do have a drawer full of early mil spec tubes, but none are nearly as big as a 12AX7. All small stuff.