Another 124 build

[BobW]

My opinion only, but believe this is one of the places in his circuit where HAD may have used a spectrum analyzer and his ears to dial in the cathodes. Once he found the sweet spot, using a trimmer, then went through numerous resistors to find the exact value.
Was it really the resistor types or just the exact value he needed, or both? I understand carbon composition, CC, resistors have no inductance except in their lead length, were used in RF circuits in the past, and sound great in Fenders, yet HAD never used them.
Based on Gil's and many others observations into real Dumble's, it seems HAD favored the CF at the cathodes. Now whether or not they will work for me, is another question. Maybe if I dial in the values that sound best, to me anyway, then play with MF, CF, and CC at the cathodes may help me understand why a particular resistor type does or does not. This probably was already answered in a prevous post, but I would like to understand/convince myself there may be more to it than tolerances.

Hey, I don't consider this thread highjacked, but in fact do appreciate all the comments here.

[Guitarman18]

Thanks BobW for letting me butt in, it certainly sounds like a good area to investigate, where you may get the extra 'vibe/mojo' out of the amp. I seem to remember Funk saying that with the poly cap values, they were usually within 1% of the stated value, which doesn't leave too many options. So with resistors values at 5-10% tolerance, this could be where the really fine tuning can take place.
Would HAD have looked for different values on an amp by amp basis or would he have had a general rule of looking for say 5% under value for example, after having done the investigation on a 'test bed' amp?

ayan wrote:

I have my own copy of #124 and it really sounds nothing like the original

Gil, did you make your copy of #124 with 'meter read' or 'visual' resistor values?

[greiswig]

Based on Gil's and many others observations into real Dumble's, it seems HAD favored the CF at the cathodes.

I'm not sure this statement is entirely accurate. See https://tubeamparchive.com/viewtopic.ph ... metal+film

I'd love to know the truth about this, or whether great ears on this forum have made a comparison between MF and CF cathode resistors.

[BobW]

Didn't mean to spin it up about resistor types here. I just need to convince myself what the tonal diffrence really is, and hopefully can report back some useful info after experimenting.

[ayan]

Bob, I am not sure if I wrote CF in error, but if I did I should have typed MF. Except one Dumble ODS, all others I have ever seen or seen pictures of have had MF cathode resistors.

Gil, did you make your copy of #124 with 'meter read' or 'visual' resistor values?

"Visual" values top to bottom, and instead of the 350K (as measured)/500K (visual) trimmer, I used a trusty 100K one. Also, my cathode bypass caps were 10uF for V1 and 5uF for V2. My brightness cap is 30pF, not 15pF, and I left out the loop's 220K//250pF filter.

Gil

[BobW]

Gil,

I understood what you were saying, and swapped out the cathodes to MFs.
It was just easier, and once the amp is ready, I may go back and experiment as part of the tweak process. Thanks!

[BobW]

glitch sorry

[BobW]

Here's a close-up pic of the relays. It didn't turn out too bad, but next time I'll use 100% teflon wiring.

[BobW]

A check fit of the Rectifier / Bias and DC boards.

[BobW]

The preamp is completed. I finally have a quiet, long weekend to complete the following:

Output sockets
Rectifier board
DC board
heaters
PT

almost forgot the MV cap.

Then I'm ready for tweaks. The head cab can wait till next weekend.

[jelle]

Are you sure about the v1b and v2b output caps? check your schem...

Looks nice!

jelle

[dogears]

Good call.... But OD1 looks correct. It is OD2 that appears to be 10X too big.

Are you sure about the v1b and v2b output caps? check your schem...

Looks nice!

jelle

[ic-racer]

First grid resistor 22K? Schematic shows 33k. Also, the resistor after OD second stage coupling cap is 150k on the schematic (looks like 100k in the picture), PI trimmer 10k vs 5k, PI plates 110/100k vs 120/110k (disregard comments if these are intentional changes )

Nice build, I'm looking forward to finding out how your's sounds.

[BobW]

Thanks Dogears and Ic- for catching the OD caps, and the 1st grid values. I was initially bouncing from Bluesmaster to HRM to #124, and finally settled on the #124. For now, I'll keep the PI trimmer at 10k w/ 100k/110k values. I have the correct values in stock, but want to try what's in there for now.

More pics and sound clips to follow.

[BobW]

The chassis is completed. After checking continuity at critical points, and ramping it up with a variac, I readjusted the bias resistor to give myself a wider / useable range. B+4 and B+4 dropping resistors also were adjusted to have 198 at V2 and 194 at V1. Before V2 / V1 voltages were 184 / 166. It not how HAD adjusted the B+ voltages but it was easier to parallel a larger / smaller wattage resistor across the dropping resistors. The paralleled resistors are under 40% rating, so there should be no future problems.

I played it for a few hours / rechecked the bias. No appreciable drift. Also I do like the FET input. It adds a little more to a strat, but the normal input works better with my humbucker tele knockoff. Clips to follow soon. Now I have to build the chassis.

Thanks to all for your inputs!