Reverb in Combo Amp?

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Reverb in Combo Amp?

Post by TriodeLuvr »

I'm finalizing the panel and chassis layouts for my Marshall-ish combo amp, and I'm thinking how easy it would be to add reverb to the circuit. One more tube, a transformer and a reverb tank, all of which I already have. On the other hand, I seem to remember reading about this a few years ago where builders said it was problematic isolating the tank acoustically from the speaker in a combo cabinet. I don't want to end up with hole plugs in the panels if the reverb has to be moved to a separate chassis. Have other builders usually been successful installing reverb in a combo amp, or is it a bad idea?

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Re: Reverb in Combo Amp?

Post by JMPGuitars »

TriodeLuvr wrote:
Fri 12/25/20 1:02 pm
I'm finalizing the panel and chassis layouts for my Marshall-ish combo amp, and I'm thinking how easy it would be to add reverb to the circuit. One more tube, a transformer and a reverb tank, all of which I already have. On the other hand, I seem to remember reading about this a few years ago where builders said it was problematic isolating the tank acoustically from the speaker in a combo cabinet. I don't want to end up with hole plugs in the panels if the reverb has to be moved to a separate chassis. Have other builders usually been successful installing reverb in a combo amp, or is it a bad idea?

Jack
I do it all the time. In a head I usually screw mount directly to the cabinet. I do that sometimes in a combo too, but more often I use cardboard with the tank inside a reverb tank bag. I install with velcro and staples, so there's no issue with the screws. One important thing is to keep the tank Out jack as far away from the output transformer as possible to avoid feedback.

The demo of my Tremolo TMB Reverb has the tank in the combo: https://youtu.be/0u5fhXF8gtY

Thanks,
Josh
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Re: Reverb in Combo Amp?

Post by Bieworm »

+ 1 on that! I only have combos and they all carry the tank on the bottom. I only have 2 combos without reverb and 5 with reverb. There are some very boomy amps among them. The VK 3x10 and the imperial 2x12 are very boomy.
The tremolo TMB reverb is very balanced with very raunchy tones. Sounds 10x better than all the others together. Been playing it all day...
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Re: Reverb in Combo Amp?

Post by TriodeLuvr »

NIce! Think I'll give it a try. When you install reverb, do you just install a single "Reverb" control for the mix, or do you also use a drive level "Dwell" control?

This is what I'm considering, although the transformer would be a standard reverb transformer like Fender:

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I think this can be installed after the 2nd stage in the Marshall 2104 with no ill effects.
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Re: Reverb in Combo Amp?

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Look for the reverb from the tremolo TMB reverb on the download section. Very good stuff. Dwell is to be placed at the input. Put a 1M pot with the .001uf cap at lug 1, grid of reverb input triode on the middle lug and 3rd lug to ground
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Re: Reverb in Combo Amp?

Post by JMPGuitars »

Bieworm wrote:
Fri 12/25/20 4:57 pm
Look for the reverb from the tremolo TMB reverb on the download section. Very good stuff. Dwell is to be placed at the input. Put a 1M pot with the .001uf cap at lug 1, grid of reverb input triode on the middle lug and 3rd lug to ground
A 1M dwell pot replaces the 1M resistor to ground at the input of my reverb circuit, same as the standard 1 tube reverb version.
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Re: Reverb in Combo Amp?

Post by TriodeLuvr »

Is the Dwell pot internal, or is it a front panel control?
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Re: Reverb in Combo Amp?

Post by Bieworm »

Front or back. you want to have access to the control.
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Re: Reverb in Combo Amp?

Post by TriodeLuvr »

OK, got it. I've seen amps with that control, but it's confusing because the old Fenders don't have it. Is that just a shortcoming in the design?
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Re: Reverb in Combo Amp?

Post by Bieworm »

Naaah it's just an extra control. Nice for tweakers and I like the option. I have a vibro king so I'm used to be able to dial in different tones and reverb settings
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Re: Reverb in Combo Amp?

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Bieworm wrote:
Sat 12/26/20 2:25 am
Naaah it's just an extra control. Nice for tweakers and I like the option. I have a vibro king so I'm used to be able to dial in different tones and reverb settings
Wow, the Vibro King is a very different design from the earlier Fenders with the parallel 12AT7 driver. How can you drive a reverb tank with a 6BQ5? And that particular reverb transformer is large, at least 5 watts. Did Fender change the basic specs for the tanks at some point? I don't think the early Accutronics I've seen could ever handle that much power. No wonder it needs a Dwell control! Anyway, my power supply can't provide that much additional current at this point, so drivers like the 6BQ5 are out. Just to be clear before I start drilling additional holes, are you sure a Dwell control will be useful with the weaker 12AT7 design?

Last question - and sorry this is so elementary - I've never inspected the guts in an amplifier with reverb. Is the tank connected to the amp chassis with standard 1/4" jacks?
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Re: Reverb in Combo Amp?

Post by Bieworm »

2 rca jacks.
The dwell control works nice with the one tube reverb from the tremolo TMB. I instalked one yesterday... easy.
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Re: Reverb in Combo Amp?

Post by TriodeLuvr »

Bieworm wrote:
Sat 12/26/20 6:34 am
2 rca jacks.
The dwell control works nice with the one tube reverb from the tremolo TMB. I instalked one yesterday... easy.
Is there a schematic online of the tremolo TMB with reverb?
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Re: Reverb in Combo Amp?

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It's in the downloads section here on 18Watt
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Re: Reverb in Combo Amp?

Post by JMPGuitars »

TriodeLuvr wrote:
Sat 12/26/20 12:44 pm
Is there a schematic online of the tremolo TMB with reverb?
viewtopic.php?f=25&t=25550

The circuit as drawn is for this single channel 18W amp. If you're building a different amp, you'll likely have to change how it lies in the circuit.
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Re: Reverb in Combo Amp?

Post by TriodeLuvr »

JMPGuitars wrote:
Sat 12/26/20 2:28 pm
TriodeLuvr wrote:
Sat 12/26/20 12:44 pm
Is there a schematic online of the tremolo TMB with reverb?
viewtopic.php?f=25&t=25550

The circuit as drawn is for this single channel 18W amp. If you're building a different amp, you'll likely have to change how it lies in the circuit.
Thanks! I'm really surprised the 12AX7 can drive that transformer. Not much current capability. Also, that looks like a different transformer than mine. I have a P-TF22921, which I think has a 15K primary, not 22.8K. Maybe I need to buy a different one? 24K transformers are available at decent prices.

The same circuit location used in the TMB is available in the Marshall-ish amp I'm building. Only real difference is that the Master in mine will be after the FX loop.

I dug out my reverb parts this morning. Bought these several years ago on eBay from a guy who never finished his project. The tank is a 9AB3C1B, shiny-plated and made in USA. It was removed from an amp. The transformer is unused, don't know if it's original Fender or aftermarket.

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Re: Reverb in Combo Amp?

Post by JMPGuitars »

TriodeLuvr wrote:
Sat 12/26/20 3:17 pm
Thanks! I'm really surprised the 12AX7 can drive that transformer. Not much current capability. Also, that looks like a different transformer than mine. I have a P-TF22921, which I think has a 15K primary, not 22.8K. Maybe I need to buy a different one? 24K transformers are available at decent prices.

The same circuit location used in the TMB is available in the Marshall-ish amp I'm building. Only real difference is that the Master in mine will be after the FX loop.

I dug out my reverb parts this morning. Bought these several years ago on eBay from a guy who never finished his project. The tank is a 9AB3C1B, shiny-plated and made in USA. It was removed from an amp. The transformer is unused, don't know if it's original Fender or aftermarket.
I think your transformer specs are almost the same as the ones I use. And that's the same model tank I use as well. You should be fine with it in that circuit.
I am ordering the Mercury FBFRT to see how it compares. It is pricier, but for once the amp I'm working on will have matching iron. ;)
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Re: Reverb in Combo Amp?

Post by Bieworm »

I had my tremolo TMB reverb running with a 9AB2C1B from another amp. This is a medium decay tank. It was still plenty plenty.
Now it's got a new accutronics 9AB3C1B and it's luuuuussshhhh. I put in a dwell control the day after. I find myself adding more dwell during band practice. At home I like a little less.. that's why I added the B1M pot to replace the 1M resistor.
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Re: Reverb in Combo Amp?

Post by TriodeLuvr »

Thanks guys, this is really encouraging. I appreciate you taking so much time to help with this. I'll add two more holes when I cut the front panel so I can install Dwell and Reverb pots, and I'll add another 9-pin socket on the chassis. I should probably order a 24K transformer, too. I think the single-tube reverb with a single anode driving the transformer will work better with the higher impedance.

Just so you know, the reason I've been trying to nail down the reverb configuration is that I won't be able to change the front panel later. I'm working with a Valve King 112 cabinet that someone gave me, and it has a 19.5" X 2" opening. I'll be using a 0.125" aluminum front panel that same size attached to the front of a 16" wide chassis. The panel will be drilled, engraved and anodized before it's attached. Once that's done, I'm stuck with it, there's no going back. I'll be forced to either make the circuit work or use hole plugs.

Launching in 10... 9... 8...
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Re: Reverb in Combo Amp?

Post by TriodeLuvr »

TriodeLuvr wrote:
Sat 12/26/20 4:59 pm
Thanks guys, this is really encouraging. I appreciate you taking so much time to help with this. I'll add two more holes when I cut the front panel so I can install Dwell and Reverb pots, and I'll add another 9-pin socket on the chassis. I should probably order a 24K transformer, too. I think the single-tube reverb with a single anode driving the transformer will work better with the higher impedance.

Just so you know, the reason I've been trying to nail down the reverb configuration is that I won't be able to change the front panel later. I'm working with a Valve King 112 cabinet that someone gave me, and it has a 19.5" X 2" opening. I'll be using a 0.125" aluminum front panel that same size attached to the front of a 16" wide chassis. The panel will be drilled, engraved and anodized before it's attached. Once that's done, I'm stuck with it, there's no going back. I'll be forced to either make the circuit work or use hole plugs.

Launching in 10... 9... 8...
Well, I just measured the transformer with an 8 ohm load, and the primary is 28K-30K. Must be my lucky day!
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