Dumble HRM 50 clone excessive hum/buzz

[Gtsirvou]

Hello,

I recently finished a dumble clone build based on the Ceriatone overtone HRM 50 and this http://pdfelectronics.com/dumble-ods-100w-hrm/ build.

I modified the design, and i have the HRM tonestack's pots (normal pots not trimmers) as well as the OD and PAB switches on the faceplate.

I used hammond transformers and choke (except for the relay supply) and Gold lion tubes (2*KT66 for the power amp, 2*12ax7 for the preamp and a 12au7 for the phase inverter. Orange drop capacitors, silver mica for the picofarad ones and 0.6W metal film resistors throughout (vishay dale for the plates of V1 and V2, normal ones for the rest.)

I have two issues that i could not resolve that might or might not be correlated.

The main issue is the excessive hum/buzz the amp produces. The guitar signal is audible clearly, but the hum is beyond normal levels. It is coming from the preamp, since grounding the power amp input makes the hum disappear. I tried to trace the source of the hum by grounding the grids of the V1 tube (the hum is present in clean and overdrive channel, so V2 is already fed with the hum). When i ground the first stage grid, the hum remains almost intact. Then the magic happens: If i ground the second stage grid BEFORE the 500p//220K combination leading to the grid, the hum only diminishes by a small amount. If i ground the grid directly the hum disappears. I am pretty good at assembling and soldering but only recently began to understand the function of each element in a circuit so this looks pretty bizzare to me, as it seems that the hum/buzz is caused by the capacitor//resistor there (I changed the capacitor with no difference in the behavior). The hum/buzz also responds to the preamp tonestack, and especially the bright switch which makes it substantially harsher. It also follows changes in the Volume and Master pots, so probably not a faulty tube (i changed those too to check nevertheless).

The second issue is that the V1, V2 and V3 plate voltages are much lower than the values that have been posted around this forum (i.e. 170V instead of 190V DC for the first stage of the V1, 190V instead of 280V for the PI and so on). The PT (hammond 290FEX) is working correctly as it is providing 430V DC to the power amp. I can only think that these lower voltages are because the amp is never truly idle, as the hum implies a plate current that drops the voltage (maybe i am correct on this, maybe not).

If anyone has some similar experience please enlighten me as to what the problem generating the hum might be. Thank you all for your patience and assistance.

[dcribbs1412]

Hi Gtsirvou
Pics of your build might help troubleshooting

D

[Gtsirvou]

I guess it would help, but i won't have the amp in my hands for another two weeks, as i am traveling, and I was hoping to do some research on the matter during this time. So if someone has experienced something similar before, or has the technical knowledge to suggest some possible causes, that would be more than helpful.

[wjdunham]

The 12AU7 in the phase inverter will cause the lower plate voltages there. The 280-290V values are for a 12AX7 that draws only 1mA or so. The 12AU7 draws a lot more current and will drop the plate voltages. Why not use the standard 12AX7 there?

Are you using a power transformer with a center tap on the heater windings. If not, then try a hum balance pot on the heaters in place of the 100R resistors.

Bill

www.sebagosound.com

[martin manning]

Did you modify the component values around the phase inverter or are you just substituting a 12AU7 for the 12AX7 in the same circuit?

[Gtsirvou]

The 12AU7 in the phase inverter will cause the lower plate voltages there. The 280-290V values are for a 12AX7 that draws only 1mA or so. The 12AU7 draws a lot more current and will drop the plate voltages. Why not use the standard 12AX7 there?

Did you modify the component values around the phase inverter or are you just substituting a 12AU7 for the 12AX7 in the same circuit?

I thought (for some reason, probably i misunderstood something) that these tubes were interchangeable without altering the circuit. Probably a mistake right? I did try a 12ax7 in the PI position with no change in the hum though. Probably changing to a 12ax7 will solve the voltage issue but not the hum?

Are you using a power transformer with a center tap on the heater windings. If not, then try a hum balance pot on the heaters in place of the 100R resistors.

This might be more crucial. As i said i made this amp based on the Ceriatone hrm 50 clone, and the one from http://pdfelectronics.com/dumble-ods-100w-hrm/. The ceriatone one uses a PT with a cener tap for the heaters, and then connects to the tubes without resistors to ground. The pdfelectronics one, uses a PT without a center tap (as is the one i got, the hammond 290FEX), and has two 120 ohm resistors from each heater lead to ground. However when i read the schematic i thought this part of the circuit (lower left corner in the attachment) was only for the pilot light. Since i am using a 240Volt indicator, i disregarded this section completely, so i have the heaters connected to the PT directly with no resistors to ground whatsoever. What could the consequences of this be, and what kind of resistors should i use (value and power rating)?

Thank you all for your answers

[martin manning]

The consequences of not having a ground reference for the filament circuit (no grounded CT or ~100 ohm resistors to ground from each side) is excessive hum. Add the resistors and see if that cures the problem.

While a 12AU7 will usually "make sound" when substituted for a 12AX7, it is a very different tube. Use a 12AX7 in the PI spot.

[Gtsirvou]

Thank you very much martin for taking the time to help me. Are these two resistors supposed to be of high power rating or 0.5-1W? I will install them and change the PI tube in approximately two weeks and i will report on the progress here, for future reference.

[martin manning]

You're welcome. 0.5 watt resistors will be fine.

[wjdunham]

Thank you very much martin for taking the time to help me. Are these two resistors supposed to be of high power rating or 0.5-1W? I will install them and change the PI tube in approximately two weeks and i will report on the progress here, for future reference.

Hopefully you will have it sorted out when you add the resistors as Martin suggests. Good luck!

Bill

www.sebagosound.com

[Gtsirvou]

Hello again,

Well, the resistors for the artificial center tap i had forgotten really made the whole difference. After installing them and switching to a 12ax7 as suggested the amp is perfect. Really low noise and absolutely beautiful sound, both in clean and overdrive!! Boy is it a singer...

Many thanks to Martin and wjdunham for the assistance!!

[martin manning]

Good to hear it's solved.