Taming amp gain in single channel deluxe reverb build

[martin manning]

I just finished the initial build of a single channel deluxe reverb, sans tremolo.
I'm using a classictone 40-18019 PT and the OT from a Hammond AO35....
The amp sounds fantastic up to 4-5 on the volume and it gets pretty hairy after that. By 6-7 the whole thing starts to really fall apart. It sounds like blocking distortion. I was a little surprised at this since the build already accounts for the lack of tremolo with a 330k and a 47k to ground on either side of the 220k mixing resistor, which should, in theory, tame the gain.

The circuit i built is meant for 6V6, which is what I am using.

I would try a single 100k to ground after the original 220k mix resistor. A series cap isn't needed.

I found Rob Robinenettes expected AC voltages and I'm going to go through the circuit with that reference to see how and where the signal is going south

Well those are Fender's expected AC voltages, but the '65 DR schematic is a good reference for checking them.
If they are high, I would consider adjusting the two 10k power supply dropping resistors to get the PI and preamp node voltages (X and Y on the Fender '65 DR schematic) down to the levels with the tremolo and normal channel included.

[cdemike]

I would try a single 100k to ground after the original 220k mix resistor. A series cap isn't needed.

I suspect this is the main culprit, especially since it seems that you've identified the problem as happening with too much signal reaching the PI. You may also want to try replacing the 330k resistor for a 250k pot wired as a varistor to find the value that suits you best. Just adjust it until you find the sweet spot and then replace the pot with a resistor of the corresponding value. You may even want to keep it as a master volume.

[imo1]

I was actually thinking the same as I was going to sleep last night. I’ll try the 250k pot
In reference to Martin’s suggestion of the single 100k to ground, wouldn’t that actually increase the gain from what I have now? The 47k and 330k are both parallel to ground, or does the 220k between them affect how they interact?

[imo1]

I just finished the initial build of a single channel deluxe reverb, sans tremolo.
I'm using a classictone 40-18019 PT and the OT from a Hammond AO35. The hammond has 2 EL84's into an 8r load. I figured the 6V6 would be relatively close.
...

There seems to be some confusion as to whether you've used 6V6 or EL84 output valves, please clarify.

What's the OT impedance ratio, eg the DR OT is 6k6:8R?
The load on the output valves affects the gain.
It's just that EL84 push pull amps typically use an 8k load.

Additionally the rest of the output valve idle operating point affects gain, please provide the anode or cathode current, screen grid voltage, Z node voltage, and OT CT B+ node voltage.

...
I used a 47r on the feedback tail.

The schematic in post 1 shows 100R. It would be helpful to clarify that the schematic you posted doesn't match your build, and to note what the differences are.

I went through the amp with a 37mA 1k test signal...

Unfortunately there are so many typos etc this whole post is gobbledygook, please repeat it using the TP references as per the Fender original. By that, we've got a better chancevof making sense of what's going on.

I am using 6V6's in the amp. I have not tested the windings on the transformer, but made the assumption that since the original design(Hammond AO-35) used EL84's, that the load was close to 8k.
The only change to the schematic that I posted was the change from the 100R to the 47R resistor

I biased the 6V6 to around 415V and 20mA, so basically 20mV of current

The signal injected was 37mV AC

TP25 I had 4.45V AC
TP31 I had 2.3V AC----Rob lists 1.9V AC as being the equivalant to 5.3V peak to peak. This is an error, yes? wouldnt RMS be 3.75? If so, then my PI circuit is doing more than enough to theoretically tame the gain, at least according to this reference.
TP
TP34 28.6V AC 158V DC
TP35 30.3 V AC 154V DC
TP33 64 V DC
TP 37 and 38 206V AC
TP39 11.4

[sluckey]

Rob lists 1.9V AC as being the equivalant to 5.3V peak to peak. This is an error, yes?

No, Rob is correct. 1.9Vrms = 1.414 x 1.9 = 2.69Vpeak = 2 x 2.69 = 5.38Vpeak to peak

[martin manning]

In reference to Martin’s suggestion of the single 100k to ground, wouldn’t that actually increase the gain from what I have now? The 47k and 330k are both parallel to ground, or does the 220k between them affect how they interact?

Sorry, I misunderstood what you had in there. In the first schematic I can see exactly what you meant. A single 100k would be just a bit more attenuation than that, approximately -1 dB.

[cdemike]

I misunderstood the first post that included your voltages and agree with Martin that your preamp voltages may be too high, especially since the signal coming out of the last gain stage being higher than spec. That said, I still think the difference likely lies downstream of the 3rd gain stage. The Fender document from which these AC readings came indicates 20% tolerance for AC readings, and the ACV at TP25 is only 17% higher than spec. At the phase inverter, that difference becomes 21%, which exceeds that 20% threshold, albeit only slightly. If you replace the resistor in question simulating the last triode of the normal channel to-spec, you'll get about a 6% reduction in signal strength, which returns your signal strength to within-spec. Reducing that resistor to 100k will reduce your signal by about 29%. It will be an imperfect model of that unused triode in any event since the resistor will be comparatively linear versus the triode, so I think just find a value that suits you there.

It does sound like you may be getting more than 21% more gain than you'd like, and the signal ratio between the speaker jack and the OT primaries as shown in the reference document is pretty different than what you're reading. It's possible that's a measurement/instrumentation error, but you may also have an issue with the OT impedance ratio that's causing some of this distortion.

[sluckey]

I just installed a 100K master volume pot in my single channel DR. Lets you control how much signal is sent to the PI. Works fine for me. Take a look...

https://sluckeyamps.com/tdr/tdr.pdf

[didit]

Hello --

Wondering what type of tube is presently installed in the PI position? Either V4 or V6 depending on reference schematic.

Best .. Ian

[pdf64]

...
I biased the 6V6 to around 415V and 20mA, so basically 20mV of current

...
TP 37 and 38 206V AC
TP39 11.4

So your HT voltage is higher than the Fender schematic you're referencing.
I guess the Z HT node voltage will be higher too.
More voltage on the screen grids means a higher stage gain for the 6V6.
The OT voltage ratio is higher too, so a higher impedance load on the 6V6, so yet more of a gain increase there.
Higher HT voltages generally will increase all anode voltages and currents, so higher gain operating points for all preceding stages.

None of the above on their own will make much of a difference, but culmulatively, with all cascaded, I think the system gain will be a notch or 2 higher, as per your observations.

[imo1]

I just installed a 100K master volume pot in my single channel DR. Lets you control how much signal is sent to the PI. Works fine for me. Take a look...

https://sluckeyamps.com/tdr/tdr.pdf

Thats nice! Its good to see a similar circuit to mine in action. Out of curiosity, what does the amp sound like with the master on 10? Mine sounds terrible. Regardless of whether i implement this or not, I would like to have a somewhat usable amp through the volume sweep.

[imo1]

...
I biased the 6V6 to around 415V and 20mA, so basically 20mV of current

...
TP 37 and 38 206V AC
TP39 11.4

So your HT voltage is higher than the Fender schematic you're referencing.
I guess the Z HT node voltage will be higher too.
More voltage on the screen grids means a higher stage gain for the 6V6.
The OT voltage ratio is higher too, so a higher impedance load on the 6V6, so yet more of a gain increase there.
Higher HT voltages generally will increase all anode voltages and currents, so higher gain operating points for all preceding stages.

None of the above on their own will make much of a difference, but culmulatively, with all cascaded, I think the system gain will be a notch or 2 higher, as per your observations.

Yes, I took this into consideration when I was measuring. The thing I don't understand well enough is what the disparity in OT would do to the sound. I am seeing a pretty similar AC voltage(11.4) at the speaker. Part of what I am hearing is probably transformer saturation, yes? I don't think I would mind that. I think I am getting some nasty blocking distortion when I turn the volume up past around 6 or so.

[imo1]

Hello --

Wondering what type of tube is presently installed in the PI position? Either V4 or V6 depending on reference schematic.

Best .. Ian

Ian! My dear old friend Ian McClagan would say "Us Ian's gotta stick together!" so passing that down the line in his honor.

I have 2 old stock At7's I was using in the PI. I ordered all new JJ's for the build, but am 1 short, so had to dig into my tube boxes. I am going to try the JJ in the PI position

[imo1]

Rob lists 1.9V AC as being the equivalant to 5.3V peak to peak. This is an error, yes?

No, Rob is correct. 1.9Vrms = 1.414 x 1.9 = 2.69Vpeak = 2 x 2.69 = 5.38Vpeak to peak

Typical Ian error. I fiddle with this stuff just enough to miss these kind of things. Totally clear now, but did not catch it

[sluckey]

Out of curiosity, what does the amp sound like with the master on 10? Mine sounds terrible. Regardless of whether i implement this or not, I would like to have a somewhat usable amp through the volume sweep.

I mostly played the amp with the MV on 10, but the volume pot was down below 5. I only play at home. I cranked it wide open a couple times in the shop just to see how loud it would get. Wide open added a little hair to the sound but nothing I would say sounded terrible. I ran 6L6s for a while but eventually switched back to 6V6s. I used MM iron and a reissue Jensen P12N speaker.

There's more info, including how I dealt with the extra gain, on my website. The 100K MV was my final fix for the extra gain due to the missing tremolo intensity pot in my amp. Works fine.

Sounds like you may have other issues. I'd sub in a proper OT for starters.