Everything You Ever Wanted to Know: Pictures & Diagrams

[mat]

Thanks so much

[Chris G]

Hi Gil,
Thank-you very much for these photo's!!!

Chris G

[Tdale]

Great photos!

Someone asked about the plate resistors looking like 100K.. I'd love to know the values too.. I had a schematic with 100K resistors, but was told that they should be changed to 220/150 180/150...

Also, the OD entrance trimmer is interesting, even if it's not 500K as written, but more like 350K... It's still much higher than the 100K we usually use... Any reason for this?

Tommy

[dogears]

Thanks Gil!! A few surprises under thar goo fer sure...

However, my suspicion over the use of large V1 bypass caps is confirmed. I guess the old boy used the larger pre OD net values and drive pot to smooth the 100k plates?

Your schematic shows 120k feeding the trimmer. Your description mentions 220k. I think the picture shows 120k but I can't tell for sure.

Good question on the carbon feedback resistor. Did anyone measure it? I suspect it may be a bit high?

Anyway, thanks again! Great work!

[dogears]

Tommy, this amp is far from the ACP link 70s amp. 100 plates are fine. Just when the amp is designed as a system. The advice to change them is sound. In order to make the small plates work, you need to tweak other areas (as evidenced by this build)

Great photos!

Someone asked about the plate resistors looking like 100K.. I'd love to know the values too.. I had a schematic with 100K resistors, but was told that they should be changed to 220/150 180/150...

Also, the OD entrance trimmer is interesting, even if it's not 500K as written, but more like 350K... It's still much higher than the 100K we usually use... Any reason for this?

Tommy

[Normster]

The OD input is quite a bit different. Notice that the trimmer is wired as a simple rheostat, not a potentiometer. I didn't do the math, but I suspect that the total signal that makes it to the grid is actually hotter with this series circuit than with the series/shunt we're used to seeing.

[tonelab2]

Thanks Gil, very kind for sharing. Much appreciated by everyone.

[dogears]

Hmmm..... Gil mentions it had 100k to ground. I wonder if the "NC" on the diagram is accurate. Interesting stuff.

The OD input is quite a bit different. Notice that the trimmer is wired as a simple rheostat, not a potentiometer. I didn't do the math, but I suspect that the total signal that makes it to the grid is actually hotter with this series circuit than with the series/shunt we're used to seeing.

[oldmacman]

If there's no connection to ground for the voltage divider into the OD stages, the grid on V2A would have no reference to ground, and the circuit wouldn't work.[/list]

[ayan]

Thanks Gil!! A few surprises under thar goo fer sure...

However, my suspicion over the use of large V1 bypass caps is confirmed. I guess the old boy used the larger pre OD net values and drive pot to smooth the 100k plates?

Your schematic shows 120k feeding the trimmer. Your description mentions 220k. I think the picture shows 120k but I can't tell for sure.

Good question on the carbon feedback resistor. Did anyone measure it? I suspect it may be a bit high?

Anyway, thanks again! Great work!

A couple of clarifications:

1. The diagram is not mine, Scott, it's Billy's. However, I did indicate in the post that the pre OD fixed resistor value was 220K and so... it really was 220K.

2. The trimmer may be shown as "NC" on the diagram, but I did say it measured 100K to ground so there shouldn't be any ambiguity there: it is grounded. Besides, as oldmacman pointed out above, V2A wouldn't really work if its grid was not referenced to ground -- no self bias would be possible for that stage.

3. The feedback resistor cannot be measured in circuit with any accuracy. However, its value is clearly 4.7K from the pictures, which is totally consistent with the ODS design.

4. Don't know if anything (10uF cathode bypass caps in V1 or 250K drive pot) was used to smooth anything that would otherwise be harsh. However, harsh was one thing that amp was definitely not, and it was not bass heavy either.

Cheers,

Gil

[keithrick]

Gil, A big thanks to you and Billy for your hard work! I feel like I am standing on the shoulders of giants every time I come to the forum!

[HiGain]

Thanks so much. Your sharing tone is commendable.

Jake

[Tdale]

It'll probably never happen, but imagine what it would be like if we could arrange a Dumble-clone gathering somehwere! A weekend where everyone from all over the world, could come, and bring their clones.

We could have some of the famous Dumble players to test the amps, and maybe have a "closest to the real thing" prize!



Tommy

[BobW]

Thanks Gil and Billy for the work, much appreciated. My hats also off to Dogears for generously offering suggestions and tweaks. This is a wonderful community.

['67_Plexi]

Thank you for sharing...interesting to know how HAD responds to this, I'm sure he knows what's going on in this community.
Trying to hide anything always makes people more determined anyway. I see nothing in there that could be considered a proprietory design.
I think his ears and ability to be able to interpret what a player wants into a flexible circuit design is his most endearing factor biggest factor. How can you patent that ? I think he must have got better at building cabs ot outsourced, that one seems pretty nasty. Not exactly Mojotone or Swanson quality or even Marshall quality.

B+ of 440V on the plates is very consistent with that particular model number of TR power transformer. I've worked on more twins than I care to think about. You may gain an extra 5 to 10 volts of so with new caps, but that's about it. As the caps become formed it always seems to drop back to 440V anyway !!