New build, OD problem, Pics added

[Bob-I]

I built this amp about a year ago but got busy before it was completed and just finished it this week.

The clean side seems to be fine, a tad thin but I can tweak that later. The OD side is terrible. To my ears it sounds like a starved plate OD, really brash and brittle. I've tried known good tubes, and swapped them anyway. Crappy cell phone recording attached, first half is clean, second is OD all knobs on 50%, with a P90 bridge.

The schematic is also attached except for leaving off the deep and mid boosts and the primary filter stage is a can cap. I added a reverb circuit as well but it's not in play yet.

Voltages...

V1a plate = 186V
V1a cathode = 1.8V
V1b plate = 188V
V1b cathode = 1.85V

V1a plate = 201V
V1a cathode = 1.7V
V1b plate = 206V
V1b cathode = 1.8V

PI
Plate 1 = 255
Plate 2 = 251
Grid 1 = 36
Grid2 = 38
Cathodes = 65
tail = 61

Power tubes (6L6GC)
Plate 1 = 390
Screen 1 390
grid 1 = -39
bias = 34ma

Plate 2 = 390
Screen 2 = 390
grid 2 = -39
bias = 35ma

Waveforms using a 1K input look as expected, symmetrical rounded off clipping.

Thoughts?

[dcribbs1412]

Hi Bob
real quick
this is a HRM?
where are the TMB trimmers set?
clean sounds OK you using PAB with OD?
plate and screens seem low
Is B+ a little low

D

[Bob-I]

Hi Bob
real quick
this is a HRM?
where are the TMB trimmers set?
clean sounds OK you using PAB with OD?
plate and screens seem low
Is B+ a little low

D

No HRM, PAB or not it has the same issue. The recording was no PAB.

The B+ is low, I built this amp for 6V6's but when I had the issue put in a known good set of 6L's and adjusted the bias.

[dcribbs1412]

Bob
gotcha no HRM
so where is the OD trimmer set?
Any pics possible
sometimes multiple sets of eyes are better than one.

D

[Bob-I]

Bob
gotcha no HRM
so where is the OD trimmer set?
Any pics possible
sometimes multiple sets of eyes are better than one.

D

I've moved the OD trimmer through the entire range, different level of terrible but always terrible.

I'll take some pics later, off to rehearsal.

[amplifiednation]

The mid switch on the schem doesn't look right.

Take some pics...

[martin manning]

The mid switch on the schem doesn't look right.

Looks ok to me... 0.002u alone or in series with 390p for 326p.

[Bob-I]

The mid switch on the schem doesn't look right.

Take some pics...

I didn't put a mid switch in.

[dcribbs1412]

I don't see the NFB on V1
Are those 1 ohm for heater tap on V1?
confirm the 2.2k/5uf and 3.3k/5uf are going to the correct pins
set OD trimmer at 20k or so
maybe check OD relay a little closer.
If clean is sounding decent you are almost there

D

[Bob-I]

I don't see the NFB on V1
Are those 1 ohm for heater tap on V1?
confirm the 2.2k/5uf and 3.3k/5uf are going to the correct pins
set OD trimmer at 20k or so
maybe check OD relay a little closer.
If clean is sounding decent you are almost there

D

V1, I skipped the LNFB, i turn it off anyway. The heater resistors are 100 ohm for hum.

Double and triple checked the Rk connections.

I'll check the OD relay. I agree, almost there.

Thx for looking

[Charlie Wilson]

Is there a chance you burnt through your coaxial in the OD?
CW

[Bob-I]

Is there a chance you burnt through your coaxial in the OD?
CW

Hmmm...I'll check but not likely, it's PTFE so it's hard to burn through.

[Structo]

Are you positive you have the cathode resistors on OD1 and OD2 on the right cathodes?

I had them reversed before as well and the OD sounded about like yours.

Other things, bad cathode cap on OD.

Uh, scratch that, your voltages look good.

[Bob-I]

Are you positive you have the cathode resistors on OD1 and OD2 on the right cathodes?

I had them reversed before as well and the OD sounded about like yours.

Other things, bad cathode cap on OD.

Uh, scratch that, your voltages look good.

Yes, voltages are ok, but I haven't checked Rk.

[jam-mill]

How far up are the masters set? That 15pF bright cap can be pretty brittle sounding at low levels.