GA20-RVT Minuteman Reverb amp gutted need new circuit.

[swingarm]

Hello, this is my first post here. I was hoping I might get some suggestions on converting a Gibson GA20-RVT Minuteman Reverb to a better circuit. The original circuit sounded like crap and looked like it was wired by a child.I will also toss the transformer it uses for phase inversion. I'd like to keep the the control layout as is but would consider using different power and preamp tubes. I have looked at the deluxe reverb circuit already. I was hoping you all might have some other circuit suggestions. Thank you folks!

[eleventeen]

I did this once, unsuccessfully, on a GA-15RVT. That was a long time ago and I didn't have the knowledge or skill to execute, and, I was trying for a dual-6L6 (35-40 watt) amp. I threw a HUGE power trans in there and by the time I was done, I had eaten up way too much room with it. Myself, I would go for a fairly stock Princeton reverb and not try to to do a 2 channel amp unless your heart is set on it. Just abandon the first channel. The GA15 was a single channel amp, thus narrower chassis, thus stuff was much closer together. The thing made too much noise to be usable, but as I said, I didn't have the build chops at the time. This 2-channel one, with a wider chassis, should have plenty of room, even to go with 6L6, should you so choose. 

With that 250-0-250 power trans, you're going to be in an inconvenient spot to make the kind of B+ that 6V6's are usually fed....high 300/low 400 volts range. But it could work. If you use a voltage doubler type rectifier, then your B+ will be highish. You can try sticking with the 6BQ5 output tubes...I myself don't like them so much, but that's just my preference.

You kind of have a multi-stage decision tree to explore. Stay with 6BQ5 output tubes? Go to 6V6? 6L6? Replace the speaker w/a 12"? Single channel (princeton) style, with the ability to make add'l gain stages out of the unused first channel? Channel switching? Go to tube rectifier? The various decisions imply metalwork (to punch out larger holes for octal sockets) or woodwork (enlarge spkr hole & mounting bolts diameter in baffle board to fit a 12" spkr) ...do you want it super neat, implying that you don't want a 10x reworked parts board in there? Tube rectifier sort of implies getting a different power trans w/a 5 volt winding....and the small output tranny (judging by the footprint) is also on the small side = reduced bass response. How much do you want to spend, both in $$ and in time? Do you want to experiment and try to make something unique, are you willing to build and tear the thing apart several times? Or do you want to go for something proven and known, with perhaps minor tweaks?

Ultimately, your choices depend upon your budget and what kind of amp you like and how much build-skill you have. I like blackface Fenders with reverb and 12" speakers and I don't especially like tweed Fenders that much. And I have used the first channel of a 2-ch Fender amp maybe 3 times in 40 years. That's just me. You have a great chassis to experiment with and/or to make a real, usable amp from...but a decidedly wimpy power trans. What I can tell you is, the one thing that will make the process unpleasant is to change battle plans in the middle of the build. That will send you into plenty of blind alleys.

[printer2]

If it is a 6BQ5 thing you could always go to 6V6's enlarging the holes for new sockets or go for 6AQ5's with smaller 7 pin sockets and make a filler plate to fit in the existing 9 pin holes. The 6AQ5's maximum voltage is in the range of what this amp puts out. 300V for 6V6's would not be too bad if you go for fixed bias, you do not loose the voltage across the cathode resistor.

Before gutting the amp I would strip out the funky stuff Gibson put in there just for curiosity sake. On the Normal channel C3 is much smaller than a Fender value, the T filter before V4 has been known to suck and would be better just clipped out. Same with R14 and the T filter behind it. 

[swingarm]

Thanks for the replies fellas. I have No problem Upgrading the power tubes. In fact I have i meatier 6v6 output  transformer I also have spare power transformers too 300-0-300. The original 12" speaker will be upgraded with a alltone 1250 that I've had for years and like the sound of in other amps I've used it in. I think the 6v6 option is the way to go. What Id like is to find a circuit that is a good fit meaning Trem and reverb. The deluxe reverb would work however there must be other circuits that would work, no? Like I say I'd like to maintain the original control configuration. Printer2 I have already gutted the amp. I had no interest in trying to work on it, but thanks anyway. I will load another board with new parts and tidier wiring.









The above circuit is not my actual board. This one is off the net and has had a cap job done. My filter caps are are old and needed to be changed.

[Backwoods Joe]

If you have any room in the chasis, you could toss the phase inverter transformer and replace it with a 6C4 tube cathodyne splitter. Whatever you do, toss the OT as well. That  with a good speaker and i think you'll be surprised how good it sounds

[swingarm]

Thanks backwoods and eleventeen. This is going to take some thought. I'd appreciate input from anyone who might have some workable solutions.

Thanks again.

[HotBluePlates]

... I will also toss the transformer it uses for phase inversion. ...

If you're gonna toss the inverter transformer, send it to me! I'd like to have it; PM me and we'll arrange shipping.

Quick things to try which may help gain and may make you rethink gutting/rebuilding:

- There are 2 bridged-T-filters in the Normal channel. These will each give the scooped mids and perceived gain loss of a typical blackface tonestack, however the exact frequency response will likely be different due to the difference of values used.
   - The first T-filter is between the Normal Volume control and the grid of V4B, and consists of C6, C7, R10 & R11. The second is between C9 and the grid of V5A, and consists of C10, C11, R16 & R17.
   - Depending on the precise age of your amp, these could be discrete components or in a package that looks like a large rectangular ceramic cap with many legs. Given the individual part designations on the schematic, yours will likely be discrete components.
   - Remove each T-filter. Replace the first one with a piece of wire between the Volume control and V4B's grid. Remove the second one, replace R15 with a 1MΩ resistor, and have the non-grounded end of R15 connect to C9 and V5A's grid.


- At this point, the gain and probably mids of the normal channel will increase a lot. Taste-test for a bit and see if you're really inclined to alter the Normal channel more.
   - If you are, The Treble and Bass controls are fairly simple circuits similar to a small tweed Fender with an extra control for Treble. If you want blackface-style scoop, remove C3, C4, C5, R6 and the Bass and Treble controls. Replace with blackface tonestack circuit and parts values, and fiddle the typical 6.8kΩ resistor to ground to tweak the blackface's mids.

- You may want to change the value of C3 (0.0047uF) after the first gain stage of the Normal channel. The existing value reduces bass maybe too much depending on the setting of the Bass control. If you retool the tonestack with blackface values, this cap isn't needed (the caps in that circuit replace its function).

- Think hard about whether you want to remove the interstage inverter transformer. Existing supply voltage is fairly low and well-suited to the existing EL84's. EL84 bias voltage is just shy of 12v, which means you only need 12v of drive from the preamp/inverter. The inverter transformer will provide near-perfect phase inversion and probably also provides a small amount of gain as-is. If you don't have a hum issue related to this transformer, I'd strongly consider leaving it.

- Yank C1 and C14 (0.0047uF input caps), or replace them with larger values (like 0.01 or 0.02uF). These cause a bass roll-off at about 62Hz, which is not bad for guitar, but other roll-offs due to coupling caps within the amp will shift this frequency higher and may lead to the amp seeming overly bright.
   - If not replaced with larger input caps, you can replace these with a piece of wire.

- The Reverb channel has 1 rather than 2 T-filters, but retains the simple Bass/Treble controls. If you need more gain or more control over the amp's tone, remove the T-filter and consider replacing the tone circuit with a blackface (or your other fave) tonestack.
   - The Reverb channel T-filter is between the Volume control and V4A's grid, and consists of C19, C20, R30 & R31. If you remove these, replace them with a single resistor of 470kΩ (or larger) from the Reverb channel Volume to the grid of V4A.
   - The Reverb channel T-filter forms an impedance that works with R48 (470kΩ) to determine the mix of dry/reverb signals. If you need more reverb and less dry signal (or dry signal gain), increase the value of the 470kΩ you added to replace the T-filter.


I strongly recommend you do any of these steps incrementally, and listen to the result. You may not want all the T-filters gone; you will also get a chance to hear exactly how they impacted the amp's tone by listening after every step. You may also find you like the action of the original tone controls after they're given some room to breathe.

The drawbacks of an interstage inverter transformer in a new design is that they are relatively expensive compared to using a tube, may have limited frequency response on the very low and high end, and poor placement may result in hum. However, if they are not driven by a massively too-big signal, they offer gain with relatively little distortion, and can exhibit perfect balance between outputs.

As said above, if you don't want it, send it to me instead of throwing it in the trash. I'll build a little EL84 amp to use it.  

[HotBluePlates]

... Printer2 I have already gutted the amp. ...

Oops, didn't see this before.

In my opinion, you messed up because while some Gibson amps are not considered toneful, you will have missed the opportunity to really learn what was holding the amp back.

With step-by-step alteration, you might have found the existing speaker, reverb, trem and phase inverter/output stage to be perfectly workable as-is.

I do understand the frustration of tinkering an amp that doesn't have a tech-friendly build style.

[swingarm]

HotBluePlates,

 Yeah this amp is not a very tweak friendly layout. I have the old board but really thought I'd be better off upgrading. I purchased the amp for a bargin and have a few transformers kicking around. The OT is tiny and while the amp worked OK it was noisy and not very loud compared to other 18 watt builds I've done. Yeah I've never seen a circuit with a driver transformer. Do you think its worth trying to work into a new circuit? I'm a noob and have a lot to learn still. If you think This amp circuit is salvageable  I might still give it a go. However its an amp from 1966 and thought I might be better of pursuing a new circuit rather than changing out old  caps and resistors that have drifted off there original values, no?

[eleventeen]

 

However its an amp from 1966 and thought I might be better of pursuing a new circuit rather than changing out old  caps and resistors that have drifted off there original values, no? 

Completely depends upon whether you wish to try and see if you can make the old circuit sing, or learn about it, or produce a right-now (or soon) usable amp. Those are not mutually exclusive, but there is a time invested and hassle factor. And IF you want to proceed directly to a proven design (+/- only a few little tweaks) then selecting the design you wish.

I can only state what I would do, given that you've torched it. I think I would build a straight ahead single channel Deluxe reverb upon a 2-channel Deluxe rev board and omit the first channel, leaving the space and turrets on the board for the first channel blank. I'd try for a tube rectifier and minimally punch out for an extra 9-pin socket that you wire up the heaters but don't otherwise use. Get that working to your liking. Later on, should the motivation strike you, try to add an additional gain stage where the straight channel would ordinarily be on the parts board.

There's definitely going to be an interaction between the footprint of your new trannies and whether a full-length Deluxe parts board will fit; or whether you will be forced to go to a smaller princeton-length board. You say you've built stuff before; if it wasn't from a complete kit, I assume that you've encountered various snarkly things like how long/wide the parts board can be before it blocks access to the tube sockets and all the other ways parts and parts placement can interfere and make your life difficult.

Thee Gibson GA15 I had, I bought non-working for $25 w/no spkr but I knew I wanted to gut it and Fenderize it without trying to fix it as a Gibson amp. I never got it working quiet enough to use and I had to drop it way down my to-do list. Then one day I had to move, radically de-clutter, and I threw it in the garbage. It's a mod mod mod mod mod world.

[Backwoods Joe]

Since you've got it "gutted" check the tube sockets & replace if any question. Some Gibson's from that era were good and some were junk.

[printer2]

It would be kind of interesting doing the single channel Deluxe and maybe use the other controls for an internal SS distortion pedal channel. That or if no interest in SS fuzzies then maybe use the controls for reverb and do a mix dwell and tone thing.


I doubt I would bother with using the transformer on a build this complex in such a small cabinet unless the object is to tinker.

[loogie]

Ya, too bad.  I have one of those and all you have to do is mess with the bridged T filters.  They can even sound nice with the filters in if you mess with the tone controls.  Nothing wrong with that phase inverter.  Its seems a shame to me to gut an old amp that otherwise is working.  Like jumping out of a good airplane...

[HotBluePlates]

... Yeah I've never seen a circuit with a driver transformer. Do you think its worth trying to work into a new circuit? ...

If you did, you'd have to set it up just like it was originally. At minimum, same output tubes and biasing, similar amount of gain in the preamp.

That's what I would have done with it had you sent it to me. I'd just build an amp as much like the GA-20 as possible, with perhaps some altered preamp stuff.

... The OT is tiny and while the amp worked OK it was noisy and not very loud compared to other 18 watt builds I've done. ...

Fair enough. You could replace the OT with no other changes and probably yield more volume with a bigger-core OT.

... However its an amp from 1966 and thought I might be better of pursuing a new circuit rather than changing out old  caps and resistors that have drifted off there original values, no?

Here's a secret: caps may leak, electrolytics may dry up, but most resistors and caps (except e'lytics) probably haven't drifted much form their original value.

No tolerance band = 20%, silver band = 10%, gold band = 5%.

In the old days (maybe still), the manufacturer made a batch of, say, 100k resistors. Those that measured 95k-105kΩ got a gold band, 5%. Of what was left, those measuring 90-95kΩ and 105k-110kΩ got a silver band, 10%. The rest, measuring 80-90kΩ and 110-120kΩ might get no tolerance band, 20%.

I say might cause while the lower value band will probably get marked as 100kΩ 20%, the upper band of values may get marked 120kΩ 10% tolerance (cause 110kΩ is still within 10% of 120kΩ).

So it's not that the resistors drifted, but they were always not exactly on the center of their marked value, tolerance. And that would have been true of all vintage amps using carbon comps. I never saw anything tighter than 10% in a Fender amp until the odd Bassman a fellow on here found that had a substituted resistor of the wrong, but close, value they had available. That one resistor was 5% because it was a 51kΩ in place of a 47kΩ that should have been there, and 51k was only available in the 5% tolerance values.

Sometimes you get lucky and a manufacturer didn't "hollow-out" their higher tolerance parts like this, but making carbon comps has never been a precision process.

Anyway, most likely all parts function properly, though carbon comps may have absorbed moisture over the decades and become hissy/poppy.

[swingarm]

Hey thanks guys, Hotblueplates thanks for your lengthy and knowledgeable posts. I'm sure everyone is glad you are here. I like the building aspect and im learning slowly. Maybe I could do a new layout in the style that kevin oconnor uses (distributed power filter caps) I have some parts here. I think I'd like to change to 6v6's too. Maybe you could look at the schem and suggest some beneficial changes in addition to what you have already pointed out? I could maybe draw up a layout and have you guys check it if you would not mind. Before i do Id like to know how i could change this amp for the better. Also how robust is the trem ldr? Will higher voltage have any effect on it?

Thanks  fellas

[HotBluePlates]

... I think I'd like to change to 6v6's too. ...

Is that for tonal considerations?

I ask because you'll have to drill/punch the chassis for bigger octal sockets. And while 6V6's and EL84's use essentially the same load at the same supply voltage, EL84's have much more transconductance (Gm) which means they require less drive signal to push them to full output power. This also means they'll bias up with less voltage than 6V6's, so the swap to 6V6 will require a different cathode resistor (if you cathode bias).

... Also how robust is the trem ldr? Will higher voltage have any effect on it?

Probably not.