Reverb in Combo Amp?

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TriodeLuvr
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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.

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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.

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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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Re: Reverb in Combo Amp?

Post by JMPGuitars »

TriodeLuvr wrote:
Sat 12/26/20 4:59 pm
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.
What's your concern with the transformer you have? On paper it's the same specs as the one I use in the reverb circuit.
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Re: Reverb in Combo Amp?

Post by TriodeLuvr »

JMPGuitars wrote:
Sat 12/26/20 6:03 pm
TriodeLuvr wrote:
Sat 12/26/20 4:59 pm
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.
What's your concern with the transformer you have? On paper it's the same specs as the one I use in the reverb circuit.
When I visited the Website at Triode Electronics earlier, it showed the replacement part no. 22921 as having a 15K primary. So, I was thinking even though that's marginal for a parallel 12AT7 like Fender used, I guess it works well enough. It's definitely too low for a single section of a 12AT7 (and especially 12AX7), though. I almost ordered a 24K transformer this afternoon, but decided to measure mine first. Either the Triode Electronics replacement isn't the same as the original, or they've made an error on their page.
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Re: Reverb in Combo Amp?

Post by JMPGuitars »

TriodeLuvr wrote:
Sat 12/26/20 7:25 pm
When I visited the Website at Triode Electronics earlier, it showed the replacement part no. 22921 as having a 15K primary. So, I was thinking even though that's marginal for a parallel 12AT7 like Fender used, I guess it works well enough. It's definitely too low for a single section of a 12AT7 (and especially 12AX7), though. I almost ordered a 24K transformer this afternoon, but decided to measure mine first. Either the Triode Electronics replacement isn't the same as the original, or they've made an error on their page.
Every site I've seen says it's 25K primary. For example: http://www.oldguitarparts.com/P-TF22921.html
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Re: Reverb in Combo Amp?

Post by TriodeLuvr »

JMPGuitars wrote:
Sat 12/26/20 8:22 pm

Every site I've seen says it's 25K primary. For example: http://www.oldguitarparts.com/P-TF22921.html
Well, I happened to visit the T.E. site first. A couple other sites didn't list the spec at all. Anyway, just a temporary confusion. My measurement shows it's actually somewhat higher than 25K, so that's good.

Small signal frequency response of this transformer is also better than I thought it would be. 20Hz to 10K is flat, then a very gradual rolloff above that. I'm sure the bottom end will degrade with any significant power (I was only driving it with a couple volts), but this is good performance.

I've decided to use a 12AT7 for a first attempt. There are several possible points for output/input in the Marshall 2104 preamp ahead of the tone controls. A little experimentation will get it right.

I've started laying out the front panel in CAD/CAM. I'll use this drawing to cut the holes and engrave markings. These long panels are a pain, because my mill table is only about 12" long. I have to work on half the panel, then turn it around and finish the other half with a different drawing turned upside-down. The result is good, but it's time consuming. Here's a panel I made about a year ago for a project that's still underway. This started as a solid 6061 panel, 1/2" thick. It's been engraved, brushed, clear anodized and filled with black gloss enamel.

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Re: Reverb in Combo Amp?

Post by TriodeLuvr »

More reverb questions...

I've been studying the drive and recovery circuits for these tanks in hopes of optimizing the design before I start building. One of the problems I'm running into is that there don't seem to be any published specs for the tank. The one I have is apparently pretty common (Accutronics 9AB3C1B), but I can't find a spec for maximum power at the 10 ohm input or for average recovery voltage at the 2,575 ohm output. I even went to the Belton site after reading that they own Accutronics now, but the reverbs there aren't the same.

There are dozens of circuit designs floating around online, but they vary a lot in terms of gain and tube types. After reading about this on forums, it's also obvious that not everyone has success with the same design. One person will say a particular design has more than enough reverb, then someone else builds it and reports not enough. Sometimes this happens even when the reverb is being added to two nearly identical amplifiers. I don't want to guess at this, it even affects how many tubes will be needed. Doesn't anyone have specs for these tanks? Or at least know rough numbers from experience?

Also, would it be possible for some kind person to post a pic or a sketch of how they mounted their tank in a combo amp? I don't know the best way to do this yet.

Thanks!
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Re: Reverb in Combo Amp?

Post by Daviedawg »

Sadly I am no help on the combo front. But I built a stand alone reverb based loosely on the Fender VTR63. The transformer is fed from the pentode of a 6U8 and picked up by a triode in a 12AT7. The 6U8 is running at the same voltages as it does in my amp circuits.
The transformer is a low wattage audio transformer which is the same as the choke in the circuit. My tank is a 9AB3C1B like yours I think.
It has run many hours in virtually constant use for more than six years so that combination is proven to be robust.

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Re: Reverb in Combo Amp?

Post by JMPGuitars »

The most important thing with reverb tank mounting is to keep the tank output jack away from the amp's OT.

I've screwed them in directly and used reverb tank bags. I prefer the bags. I use industrial velcro, and staple it to the cabinet. I place the tank inside the tank bag, along with cardboard along the open side of the tank. If you don't trust the velcro alone, you can also staple the tank bag wings down.
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Re: Reverb in Combo Amp?

Post by TriodeLuvr »

Daviedawg wrote:
Tue 01/12/21 3:23 am
Sadly I am no help on the combo front. But I built a stand alone reverb based loosely on the Fender VTR63. The transformer is fed from the pentode of a 6U8 and picked up by a triode in a 12AT7. The 6U8 is running at the same voltages as it does in my amp circuits.
The transformer is a low wattage audio transformer which is the same as the choke in the circuit. My tank is a 9AB3C1B like yours I think.
It has run many hours in virtually constant use for more than six years so that combination is proven to be robust.

Dd
Thanks for mentioning the 6U8. It's a nice tube, and I've used it in other applications in the past.

I finally found a reference to all this last night, a Fender schematic that shows AC signal levels throughout the amplifier. The numbers are highly dependent on frequency, but it’s a guide. I also simulated the Fender circuit to verify the gain in each section and to confirm maximum available levels.

It turns out that the amplitude driving the 24K:8 ohm transformer is higher than a single section of a 12AT7 can produce. It simply doesn’t have sufficient current capability (clearly the reason they used two sections in parallel). A single section of a 12AU7 could provide the current, but it lacks sufficient gain. On the recovery side, Fender used a 12AX7, so neither the 12AU7 or 12AT7 have enough gain to replicate Fender’s signal voltages.

I also modeled the Marshall front end from the Low input through to the tone stack. At the circuit locations where it’s most practical to pick off and inject the reverb signal, more gain is needed to duplicate Fender’s numbers than can be accomplished with a 12AT7. The bottom line is that Fender’s reverb functionality can’t be cloned into the Marshall with a single bottle in the reverb “module.”

After futzing with the simulator for many hours, I’ve decided to take a two-pronged approach. First, I’ll build this to use a single 12AT7 around the reverb tank. That won’t duplicate Fender’s drive amplitude or recovery gain, but it might still be useable. Some builders have reported satisfactory results with this configuration in other amplifiers. If it turns out not to have full effect, I'll go to plan B. First, the 12AT7 will be changed to a 12AU7. Then, I’ll bias one of the cathodes to +20V or so using a Zener. This voltage will be used to power a dual, low-power opamp. The opamp can easily compensate for the low gain of the 12AU7 in both drive and recovery, effectively matching Fender’s numbers for operation of the tank.

I know this might sound complicated, but the entire opamp circuit can fit on a PCB the size of a postage stamp. Boards like this are currently being sold on the ‘Bay for a couple dollars each, and it's really a straightforward approach to getting this done without re-engineering the entire signal chain. I also won't have to cut more holes after the amp is built, which is one of my primary goals. :)

Jack
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