New ef86/6BR7 Special build complete question about settings

[chopstuck]

I have just completed my first D- style build last night and have a question about the OD channel settings.

The 86-special is very controllable in the clean stage. When I go to OD though it's really easy to get splatty distortion. In fact when I increase my picking attack it gets splatty on the attack of the waveform.

Is there a way to adjust tout this type of distortion or is there a problem with my build ? I'm really in the dark about using a pentode as an OD stage.

I used a Brimar 6BR7 pentode which is supposed to be equivalent.

I also have a little squeal when the volume is dimed. I will work on my lead dress to sort that out.

I have all the pots panel mounted. Is the OD tone a set and forget type of control ?

[Structo]

Can you post the schematic you used?

[chopstuck]

Here it is as well as my build. I did use a 6BR7 instead of an ef86 with the required pinout changes


Are there any other signal paths that need to be shielded ?

[rogb]

How are your voltages compared to the schematic? Are you using the Hammond 269/369?

Reason I asked is looking at your rectifier. The AX84 power amp specifies a 2 diode fullwave recto. Just wonder if the voltages on your B+ rail are in spec?
I they are not right there, then you will have some tweaking to do.

I do know ALL about these rectos

[chopstuck]

Thanks for the replies, I'm a junkman by nature and have lots of old transformers. This PT is out of an old Magnatone Varsity it was 220-0-220 unloaded. (I used a new Allen 10B OT with multitaps).
I used a regular SS recto and got within 5V of spec.

I think my problem was with a parasitic oscillation. I put my input resistor right ON the pin of V1a, took off my artificial CT on the heater, moved a V1b plate wire a bit and the splat went away.
Hmmm.

What little squeal I have left goes away when I touch ground on the guitar.

Distortion is more subtle and I think I may now know why another of my amps sounds "funny".

[chopstuck]

OK guys, the sputtering is back again. It sounds sort of like a BB bouncing on the speaker cone. It does this with quite a few speakers but only in the OD mode.

The clean mode sounds great and can get plenty dirty by itself.

Am I looking at blocking distortion ? It happens with the vol at 12:00
and the OD levels set at minimum and just gets more fuzzy with increased gain. Almost like a fuzz octave...no octave though.

[10thTx]

Have you tried another tube? Any chance the OD tube is bad?

With respect, 10thtx

[chopstuck]

Yes, I tried a couple of known good tubes.

The only value out of line with the published schematic voltages is my pentode plate at 127v instead of 105v.
The screen is at 31v. I tried increasing the dropping string feeding that node from 82K to 110K to get the right voltage , fixes the numbers but it really doesn't change much tone wise.
Still chopsticking...

[chopstuck]

Well my printer garbled the reduced image up and it looked like 31v. That's what I measured. The full res schematic shows a 51V screen when the plate is 105v, cathode at 0.8v.

When the string was the stock value everything was right EXCEPT the screen measured 31v the plate 128v and the cathode 1.5v. all other places seemed +/-2.5v.

I have been juggling plate and screen resistors and got the screen to 53v with the plate to 101v and the cathode down to 1.1v

Still does not sound great in OD mode. So far I'm not sure what I'm supposed to be hearing. I do know it doesn't sound much like the clip in the original thread.

Maybe I just can't tune this here radio.

Maybe this amp need a user's manual for getting the specific tone we are looking for ?

[10thTx]

If you have a 5879 pentode handy ....... you could try that instead.

It's a good sounding tube with low microphonics. It has less gain than the EF86. I have used a 5879 in the OD section of my Tweed Overdrive Special and like it alot.

with respect, 10thtx

[chopstuck]

Well I've traced out my amp, no flaws that I can see so it must be my misapplication of the 6BR7. I believe I'm getting too much gain.
My cathode voltage is up to 1.5v vs the 0.8 specified.

The tube behaves differently when using standard EF86 values

I will try to find a proper values for making this tube behave more like an EF86.

I remember now that I had similar trouble when using it as V1 in another pentode design. It worked but not quite as well as the tube it replaced.

10thTx, will the 5879 work with the standard EF86 values (I have a few) or do they need changing too ?

I built your 56T and love the tone of the arrangement, as well as 6BM8s.
It's in my top three favorite builds !

[Structo]

Does the EF86 really require that 1M grid leak resistor to operate?

[John_P_WI]

I have not played with the EF86 extensively, but I do know they are sensitive to the load on them, ie tone stack etc. Check the output side as well as down stream grid leaks.

If it is splatty, it sounds like something is charging up, I'm guessing there is a missing / bad ground, bad cap or improper or missing grid leak or loading.

Good luck

[10thTx]

10thTx, will the 5879 work with the standard EF86 values (I have a few) or do they need changing too ?

IIRC, DaGeezer used EF86 values (as a starting point) with the 5879 when he designed the HoSo56 (56T).

I don't know if the 5879 will resolve your issue, I just would be tempted to try it since the other tube doesn't seem to be working. And I know the 5879 is a nice musical tube.

With respect, 10thtx

[chopstuck]

OK, I gave up on the 6BR7 and rewired the socket for an EF86 for a baseline - scavenged from another amp.

Darn, the readings are still off.

I have 54v on the screen 136v on the plate and 0.84v on the cathode.
Looks like about 30v high on the plate.
Current draw is on the money.

Is the node between the 82k plate and 820k screen supposed to measure 250v ? I only get 205v.

The amp sounds more controlled with less over the top gain.

I wasted a lot of time juggling screen and plate resistors before.
Though I'm wondering about the schematic with the 56k resistor in the string before node C. Was that part of the original amp or was it part of the process of getting the voltages to pan out ?