65amps SoHo

[dorrisant]

Yes, 6.8k is correct. I lifted a leg on a few to confirm the measurements...

[arjepsen]

yeah that makes sense.
How about the whole bump-circuit?
I cant exactly make it out in the pictures, but I am suspecting that when the bump is engaged, the 470k in series with the pot is paralleled with the 6k8, making it a kind of variable tonestack lift, with a minimum value of 6k7.

yet another question:
In my home-built version of the Soho, I have an issue with squeeling that happens when both volume and treble are maxed out - this is primarily on the normal "channel". It seems to be an instability around the cathode follower. You haven't experienced anythign similar with yours, right?

[dorrisant]

yeah that makes sense.
How about the whole bump-circuit?
I cant exactly make it out in the pictures, but I am suspecting that when the bump is engaged, the 470k in series with the pot is paralleled with the 6k8, making it a kind of variable tonestack lift, with a minimum value of 6k7.

yet another question:
In my home-built version of the Soho, I have an issue with squeeling that happens when both volume and treble are maxed out - this is primarily on the normal "channel". It seems to be an instability around the cathode follower. You haven't experienced anything similar with yours, right?

I went back through all of the resistor values, in particular the 470ks and the 6k8 that you mentioned. All of my connections were checked too. I updated both the schem and the layout on the previous page. I am 99.9% sure that what I've posted is correct. If you have some time, please take a look and let me know what you think of the updates. I'd like to post this over at the Hoffman forum just to complete that thread.

As far as instability, I would have thought by looking at the lead dress in mine, that I would have trouble with oscillating too. But no, not really. This thing gets a bit to mushy for me when I turn up the gain (volume pot). I like loads of gain a lot of the time. I wouldn't say this amp gets brutal, but it has plenty of gain here for most of what I'd like out of it. So it has more distortion than I'd ever need before it ever gets to the mushy point I won't use it go past.

All in all, I really do like this circuit. It is a bit more versatile than the Hoso 56 I built back when the guys were brainstorming over at the Hoffman forum. I'm tempted to build a clone and sell this one. I don't mean to sound like an ass, but I think I could build the same circuit into one of my crazy concoctions and be much happier with what/how I'd build.

Also, this thing didn't act exactly as I expected it to right out of the box. I had to learn how to set it up. But after a little getting to know it, I found several tones that I really liked and then started playing with tones adjacent to those and it really unfolded from there. It is a very versatile circuit. It is a shame that 65 isn't building them at the moment. What a shame for 65... They get it going again all of the odds. Then get stifled when they gear up to do more. Perfect example of a really good company building a really good product with their foot firmly in the door of the amp business and they go silent due to extraneous BS. Sad really. I wish I could back away from where I work to help them out.

One last thing that I forgot to write down was the unloaded PT HT voltage. I am pretty sure it was 660vac. (Had a chance to check It. It is 630vac.)

Let me know if you have any more questions about it. It is not going anywhere at the moment.

[arjepsen]

Thanks for updating the schematic, and adding voltages - that's awesome!
I got questions

1) the blue thingimabob on the input.... text on it says RIE / 104M, so I might also assume .1µF, but can that be true? wouldnt that shunt way too much of the signal to ground??

2) I do see there's two 68k's on the input. Wonder why they didnt just use one 33k. doesnt really matter though.

3) I'm assuming C9 is 220/250pF, not 1000?

4) I'm still very unsure about the bump level. As it is drawn in your latest schematic, when the bump is activated, there's a 470k in series with the 500k pot (variable resistor) to ground - so at least 470k to ground. If I do the same in my circuit, then the bump level has no effect. are you absolutely sure this is correctly drawn? would mind taking more pictures of the relay arrangement, and maybe measure thoroughly the resistances to ground from the bass pot junction in both normal and bump mode?

5) R28 and C19 on the cathodes of the EL84's.... I think they make be different values??

6) You've drawn the diodes D1-D4. But are they really in your circuit? I can't see them in your pictures?
Also, I haven't seen the R23-C17/C18 arrangement in the pictures of other Soho's I've seen.... it seems like "static cut-control"? maybe serving a similar purpose to the diodes?

7) The circuit seems to me to be heavily inspired by the marshall 18watt, but I think I read somewhere that they used a 9k OT, instead of the more regular used 8k?

8 ) The VVR circuit seems a tiny bit different than what I usually see.
Normally, there would be a 10R/3W resisot between the source of the mosfet and the anode of the zener.
But in your schematic, and also as far as I can tell from your pictures, the zener goes dicrectly between gate and source - is this correct?

9) can you confirm that the value of R32 is 100R, not 10R?

Again, thanks for your information!

[dorrisant]

1) It is connected from the jack ground to the chassis ground on the front panel.

2) Yep, that's the way they built it.

3) Nope... it is 1000p

4) Yes, I'm pretty sure. I'll do this when I pull it back out again... may be a while.

5) Nope. Single resistor and cap for the EL84 cathodes.

6) Yes. Look at the layout, you can see them under the board at the 470ks to ground.

7) Again, I'll check and report the OT impedance when I Get it back out.

8 ) Yes. I pondered over this too. I checked with old schematics for the VVR and Power Scaling. It is within reason as built.

9) Yes, you are correct. It is 10Ω. Updated the schematic. Had it right on the layout.

[arjepsen]

Thanks for the continued information.

Point one, I think the schematic shows the cap as going from signal to ground, not ground to chassis? But I could be reading it wrong.

I think you may have misunderstood me on point 5 - the values of the cathode resistor and cap....
In the schematic you have set it to 8k2 and 47µF.... Im fairly certain the resistor is 125R in your pictures...
And I think the cap in other models I got pictures of, was 150µF.

I am wondering if I am misunderstanding how those relays work.... would you have a chance to either take a pic, or simply write their model/make/version?

[dorrisant]

Point one... Nope... Signal is on the other (left) side of the jack. The right side supplies ground to signal path when the cable is not inserted.

As regards the power tube cathode set... You are correct. Another case of copying and pasting to get the topology right, then forgetting to correct the values. And yes it is 125R || 150uF.

I'll have to do a breakdown of the relay circuit and that will probably help. Just can't get to it at the moment. Please be patient, I'll try to get around to it by this weekend.

[arjepsen]

Awesome, thankyou
Heh - it's your time and your decision - any impatience on my part is irrelevant

[dorrisant]

Awesome, thankyou
Heh - it's your time and your decision - any impatience on my part is irrelevant

It was just a forewarning of my situation... No implications intended.