New DR103 clone build - passing farty low frequency signals only

[Ceece]

Hi,

I have just finished a build of a DR103 clone based on the schematics at hiwatt.org

Initially I wasn’t getting any signal through at all as I had wired in the side mounted can cap with one of the positive and negative terminals swapped

Now with that fixed I am getting this weird behaviour where no signal passes until the master volume is about 1/4 to 1/3rd up (should be quite loud at this point by all accounts) and at this point it will pass farty low end sound through when picking hard on the guitar, I was using a signal generator originally but the presumably much stronger input signal seemed to go through fine at lower master volume and didn’t reveal the issue like playing a guitar through it does

I’ve checked all my earths and voltages and it all seems to be within expected value ranges - only thing I noticed is the plate and screen voltages are identical in all tubes as opposed to ~100v difference in the values that M Huss posted for his build.. but I believe from researching this may not necessarily matter?

I have Also tried disconnecting NFB and had no change to the behaviour

has anyone heard of something like this and can point me in the direction of things to look at?

Thanks in advance :)

Audio - https://voca.ro/1jQnaY4uxuH8

[PRR]

Welcome.

Link to the schematic and layout you used. Say what changes you made (I am tired of finding out a week later that the 6V6 is really a Russian radar tube). Report B+ and plate voltages (first). If you have a "verified" voltage chart, link to that.

[Ceece]

Apologies -

Here are the schematics/layouts
https://hiwatt.org/Layouts/DR103hiwattlayout.pdf
https://hiwatt.org/Schematics/DR_Pre4Input_v1a.pdf
https://hiwatt.org/Schematics/DR_100wPS.pdf
https://hiwatt.org/Schematics/DR_100wOutput.pdf


RE: Tube types - All ruby EL34s  bought as a matched quad - Although I do notice now that one of the 4 has the halo getter connected via the opposite side of the plate - I suspect this doesn't matter but it is something that stood out when inspecting for signs of a bad tube.

Voltages:
494V on plates
B+: 496V

~ -70V on grids with input open but no incoming signal

No changes have been made to the circuit other than the currently disconnected NFB - no difference to the symptoms when it is connected or not either way

I guess I should mention I am running off of the 240V tap on the PT and the PT and OT are the MM 100W partridge clones - HI100-P and HI100-O on this page https://www.mercurymagnetics.com/make/hiwatt/

[sluckey]

~ -70V on grids with input open but no incoming signal

No wonder you're getting little or no sound. The output tubes are cut off. -70 is waaaay to high for EL34s. The mhuss schematic you posted shows -38V. That's what you have to put on your grids too.

How are you getting -70V? What power transformer" Post the PT specs.

[Ceece]

https://www.mercurymagnetics.com/2018/wp-content/uploads/shop/specsheets/HI100-P.pdf

I am tracing the bias circuit now and I can't find anything obviously wrong -- this was one of my earlier thoughts - too cold bias but I couldn't identify anything wrong with the circuit just tracing it out and checking continuity etc.. The only thing that is obviously different is the layout shows the 1k resistor on the opposite side of the diode to the schematic -- Reasoning it out in my head I don't /think/ this would make any difference so not sure where to look next - seems its just the full bias voltage instead of the 1/2 it should be

[sluckey]

Remove the EL34s and don't put them back in until you have -38V on pin 5.

OK, that PT puts out too much voltage on the bias winding (grey wires). Right now you have a 1K resistor connected to the bias diode. Replace that 1K with a 10K and recheck the voltage on pin 5 of the output tubes. If voltage is still greater than -38V, try a 15K. If voltage is less than -38V with the 10K, then try a smaller resistor. The idea is to use a resistor that will give you -38v (or very close). You have to experiment to find the right resistor size.

[Ceece]

Thank you I will give this a go now

[sluckey]

Let us know how it turns out. Hope you have enough resistor values for this experiment.

Once you have about -38V on the EL34 grids you can put the tubes back in and see if you have sound.

[Ceece]

Hello,

Bit of a delayed response as I've just spent a good 20 minutes with my jaw on the floor playing this bad boy

This was exactly the culprit :) Got it down to -40 on the grid which seems to be close enough - didn't really have resistors to fine tune beyond that anyway - Now that it is at a working state I will probably add a trim pot there to fine tune

One last problem to track down now which is when I turn the master turns past about 10 o'clock it gives off a weird high pitched noise for a moment - I'll probably try to track down a NOS mullard for V1 as well - the one I've got is a tad noisy and I'm sick of rolling chinese/russian tubes in my amps trying to find a decently quiet one

Thanks for the help :)