Rocket Reverb - Less Headroom than Expected

[jhaas]

Do you have a 12AY7 handy?

Yes I can give that a try.

How are you running the tone stack? Have you tried adjusting the midrange?

Stock tone stack with the addition of a 25KL Midrange pot in place of the 10K resistor

If V1 voltages are low, you might adjust the dropping string to bump them up.

I've done that to some degree, but V1 is still low (75V - see below).
Current PS Rail Dropping Resistors is as follows: Choke-18K-9K1-4K7

The cathode follower is stealing current from the previous stage, adding some gain to it. To reduce compression a little, you could try increasing the 56k cathode resistor on the cathode follower up to 100k.

I'm currently running a 68K instead of the 56K.

What el-84s are you using?

two matched pairs of JJ EL84 (matched quad on order)

Do you have a scope?

Embarrased to say yes, picked up a Techtronix 453 on eBay a couple years ago, and *still* haven't learned how to use it.

Here is a full set of voltage readings. V1A plate was actually in the 80's last time I measured. I've been swapping tubes around since, and now it's down around 75V.

VOLTAGES:
AC Mains: 123.2
PT HV: 517 VAC
B+ (plates): 305
B+1 (screens/reverb driver): 295
B+2 (PI): 218
B+3 (V2A-CF): 187
B+4 (V1, V2B-Reverb Recovery): 182

V1.1: 74.4
V1.2: 0
V1.3: 0.72
V1.6: 114.7
V1.7: 0
V1.8: 0.97

V2.1: 187.8
V2.2: 114.4
V2.3: 114.8
V2.6: 126.5
V2.7: 0
V2.8: 1.23

V3.1: 292
V3.2: 0
V3.3: 2.36
V3.6: (292)
V3.7: (0)
V3.8: (2.36)

V4.1: 166.6
V4.2: 27.3
V4.3: 42.9
V4.6: 177.9
V4.7: 29.5
V4.8: (42.9)

V5 and V6:
Pin3: 9.0
Pin7: 295
Pin9: 295

V7 and V86:
Pin3: 9.0
Pin7: 297
Pin9: 295

Anything jumping out...?

[jhaas]

Update - I've used what Doug Hoffman over at EL34World.com calls a "Listening Amp" to determine where the early breakup is coming from.

For those unfamiliar, basically that entails connecting a dummy speaker load to the amp your testing and then connecting a shielded test lead to various points in the amp to feed the signal to an external amp set to run clean. In my case I use a Fender Frontman 15. This enables you to hear the signal at various points in the amp.

Anyway, the signal in my Rocket stays clean up to the PI. When I fed the Frontman the signal from just past the PI coupling caps (C10 and C11), the signal was definitely clipping.

I had the volume and tone controls all at 12:00.

So, my question is, how should I go about addressing this?
change PI values?
Shunt some signal to ground pre-PI (PIG control)?
Other ideas?

I currently have a 12AX7 in the PI and unfortunately I don't have an 12AT7 to experiment with in there.

[PlinytheWelder]

I would double check the resistors around the PI circuit ( I messed one up, but caught it with a voltage check) and just switch out the coupling caps. You may just have a bad cap...

[TheGimp]

I presume V4 is the PI.

There is 2.2V difference in the grid voltages and 10+ volts difference in the plate (anode) voltages. The grids in particular look to be off by a lot.

What is your B+ supply to the PI stage?

I'd look at the grid circuit for a bad (leaky) cap possibly causing a shift in the voltage if the resistor values measure correct.

[jhaas]

Pliny - thanks, I've double checked all the PI resistors, they all check out good. I replaced both coupling caps as you suggested and that didn't make a difference. I do appreciate the suggestions tho.

Gimp, you are correct, V4 is the PI, supply voltage is 218. IMO I wouldn't expect the voltage discrepancies I'm seeing to be sufficient to make that much difference.

I've attached the PI section of the "Compiled Rocket Voltages", and variations as much as I'm seeing seem normal.

What do you guys think?