New EF86 Special build

[sixstringer]

I've just constructed an EF86 Special using Mat's layout, schematic and BOM,
The B+ voltages measure about 8 to 10 volts low but the amp sounds very nice. The issue is, the OD side is just about a clean as the clean side, there is no significant over-driven tone to speak of. Both channels are plenty loud. The treble and bass controls don't seem to do much but all the other controls seem to work fine with plenty of range. Any ideas as to why the OD channel has no OD?
Thanks

[xtian]

Any ideas as to why the OD channel has no OD?
Thanks

You can't get any decent distortion with a layout that's that neat and pretty. Where will the dirt come from?

Seriously, dat pretty.

[Structo]

Not familiar with that particular amp.

Do you have a schematic?

For OD does the first stage cascade into the EF86 stage?

[sixstringer]

Thanks, Here's the schematic and layout I built too (with the effects loop omitted). I agree that it sounds like the 12AX7 stage and the EF-86 stage are working independent of each other. I'm going to the check the channel switch wiring to see if there is an error there. The sound files that Mat posted for this design sound great and I've had good luck with EF-86s in other circuits so I'm really looking forward to getting this sorted.
I appreciate you taking the time to review and comment on it.

[10thTx]

Here is the schematic in a GIF format

[tele_player]

Does the drive trimmer have any effect?

Edit: just noticed it's on front panel, so I expect the answer is yes.

Robert

[tele_player]

If it's wired anything like the schematic, the channels can't work separately.
Does the OD channel stop working if you remove the EF86?

Robert

[sixstringer]

Yes, indeed the OD channel stops working with the EF-86 is removed BUT after confirming that the channel switching is wired correctly and that the OD channel controls (level, drive and tone) all work correctly, I discovered that even though the filament circuit is supplying power to the EF-86, the tube does not seem to be heating up or producing any gain. I don't have a good spare EF-86, so I'll get a new one tomorrow and see if it's the tube. Fingers crossed, that's it. I'll report back with an update. Thanks again for sharing your time and expertise.

[sixstringer]

Dang, it's not the tube. I checked the component values for the EF-86 stage. the bias and plate voltages match those listed on the schematic.
Now I think it's gonna be one of those embarrassing ah-ha things.....
I'm still looking.....

[tele_player]

Dang, it's not the tube. I checked the component values for the EF-86 stage. the bias and plate voltages match those listed on the schematic.
Now I think it's gonna be one of those embarrassing ah-ha things.....
I'm still looking.....

I'd double check the values of the screen and plate resistors on the ef86, looks like they'd be easily confused.

Robert

[sixstringer]

Agreed, I checked the 820r, 82k & 820k first thing, all correct.

[matt h]

(deleted)

[sixstringer]

Compared to the posted voltages, my amp is with in a few volts (on the low side). The first value is the posted voltage, the 2nd value is my amp
Point A: 263v/257v
Point B: 257v/ 247v
Point C: 261v/ 250v
Point D: 259v/ 240v
EF86 voltages: bias: .8v/.86v
Grid: 51v/50v
Plate: 105v/ 118v

This circuit design and construction is well discussed and documented in "the EL84HRM", "MINIHRM evolution" and "EF86 Special back to EL84 HRM" threads here on TAG and also at the AX84 website. Mat's sound files prove the sweet quality of the OD circuit.

The way my amp sounds now, both channels have a good clean EL-84-ish sound with the EF-84 side delivering a slightly high gain quality than the 12AX7.... It's as though the power section is being driven by one preamp stage or the other with no cascading OD quality when the 2nd gain stage is supposed to be engaged.

[matt h]

(deleted)

[10thTx]

Here are Mat's voltages.

With respect, 10thtx