Taming amp gain in single channel deluxe reverb build

[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.

All my voltages are just where I want them. The amp is buzz free and all the controls work well. Reverb sounds good.

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

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

single channel deluxe reverb schematic.jpeg

[imo1]

I went through the amp with a 37mA 1k test signal. All pots at halfway
Ac followed pretty close until PT. At V4B grid, I was at 4.45vAC(spec was 3.8. ).
At input of PT I was at 2.3vAC(spec 1.9) and on other grid .62(spec .91)
Output of PT was 28.6 and 30.3vAC(spec 22 and 23!) and output of power tubes was 206 and 205 vAC(spec 174)
I did notice that DC on the PT was a bit low. plates were 158 and 154vDC. Cathode 64

I'm assuming that I need to drop some more gain in that area around the mixing resistor. I'm wondering though if there is something funky in the circuit around the PI, so before I go on that tangent i wanted to see if anything jumped out.

65_Deluxe_Reverb_Gain.jpeg

[martin manning]

I'm having trouble following you. Maybe refer to the TP (test point) number where you are measuring? What is "PT"?

[johnnyreece]

From the drawing and text, it appears they meant PI rather than PT in some places, and PT is short for Power Tubes in others. A touch confusing, for sure...

[imo1]

Sorry! I meant phase inverter. Was more focused on the numbers. The phase inverter gain is quite a bit higher than what Rob was getting in his circuit.

[bepone]

try 12ay7 for the first tube?

[cdemike]

I'd suggest reducing the 330k after the faux mixing resistor. As it's built now, there's an assumption that plate resistance of normal channel's second stage is infinite, making it so that the only path to AC ground is through the mixing resistor and the normal channel's last stage's plate resistor. So accounting for the parallel path to ground via that gain stage's plate (~63k) that 330k should be closer to 259k. That'll drop the signal level reaching the PI some.

Edit: I'm also seeing your NFB tail resistor is about 2x larger than the AB763 circuit I'm used to seeing. Maybe there's some strangeness happening with NFB breakdown with that different value there as well?

[Stevem]

Please clarifie what your doing better.

You can't be pumping in millamps ( MA ) of signal like you posted, millvolts yes.

Interms of the MV level are you saying .037 volts or .137 volts,

I use about .115 volts. (-10 DB ) which is about a single coil pick up output level, if your pumping in .137 then that's hot.

.115 volts is enough to drive any guitar amp to sag its power supply and attain max peak wattage.
Also as posted that high feedback resistor is leaving a lot more gain in the circuit.

Also note that EL84s have more inherent gain then a 6V6 does.

[imo1]

It is 37 mV!
I used a 47r on the feedback tail.

[Stevem]

Note that 6BQ5 PP output stage needs 10 volts less of grid to grid drive signal then a 6V6 output stage does.

[imo1]

I'd suggest reducing the 330k after the faux mixing resistor. As it's built now, there's an assumption that plate resistance of normal channel's second stage is infinite, making it so that the only path to AC ground is through the mixing resistor and the normal channel's last stage's plate resistor. So accounting for the parallel path to ground via that gain stage's plate (~63k) that 330k should be closer to 259k. That'll drop the signal level reaching the PI some.

Edit: I'm also seeing your NFB tail resistor is about 2x larger than the AB763 circuit I'm used to seeing. Maybe there's some strangeness happening with NFB breakdown with that different value there as well?

I will try reducing the resistance of the 330k. Do you mind expanding on what you are saying about the plate resistance being "infinite". I have looked at the circuit a number of times and I have no concept of what you are talking about! I know the 47k to ground is to account for the tremolo circuit not being there, but assumed that the 330k was also there to take some more of the AC voltage to ground. I assumed the 2 resistors were forming voltage dividers with the total plate impedance, which i thought was the 100k with the 63k in parallel.

[imo1]

Note that 6BQ5 PP output stage needs 10 volts less of grid to grid drive signal then a 6V6 output stage does.

Are you saying this in reference to my using of the OT from the dual EL84 circuit? The circuit i built is meant for 6V6, which is what I am using. The only part of the design that I'm understanding as affecting my design, at least from your inference, would be the OT overloading and somehow not able to handle the signal.

I guess I am confused, because my testing showed me that my phase inverter was giving me quite a bit more output than it should, and to me mind(just knowing enough to get in trouble) the issue would be to pull the phase inverter signal down enough that I'm able to get usable signal, in line with what I would expect out of a typical deluxe reverb, with a more typical distortion as I get to the top of the volume control

[imo1]

try 12ay7 for the first tube?

The 1st gain stage is operating as it should. I am trying to figure out where in the circuit the extra gain is coming from, and learn why and how to tame it.
I am building this to perform like a simplified deluxe reverb, which i think is an amazing amp, and I am building it for my younger son so he has something he can use with his band!

[pdf64]

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.

[bepone]

try 12ay7 for the first tube?

The 1st gain stage is operating as it should. I am trying to figure out where in the circuit the extra gain is coming from, and learn why and how to tame it.
I am building this to perform like a simplified deluxe reverb, which i think is an amazing amp, and I am building it for my younger son so he has something he can use with his band!

replace V1 for 12AY7 or 12AT7, the same for the PI if the rest of the amp is ok.