In the attachments the schematic from LTSpice.

• 200V is the output of nixie psu

• anode resistor adjusted according to 6N17B datasheet and Manfred's suggestions

• To get 6V out of 9 i setup a voltage divider using 1Ohm 2W resistorsse it is what i have got on my bench at the moment

heater.jpg Mainly the issue was caused by a power supply transformer feeding lower current than needed. Using this set of resistors and a proper transformer reduced the drop. Now with treble past noon and volume from noon it oscillates. What could be the issue? I have noticed that the v1b grid is a...

Manfred wrote: ↑17 Mar 2024, 10:04 For how many tubes do you need the heating voltage generation?

At the moment just 1

Manfred wrote: ↑15 Mar 2024, 14:55 The link is to a picture of the step-up regulator.
What is the circuit of the voltage regulator for the heaters?
Please attach a sketch of the schematic.

Using the Mosftet Irf830 looks like this

I have connected 9V as input of the Lt7805 regulator. Used cap 330n from input to gnd, gate to gnd and output connected with a cap 100n to gnd. The output is then connected to first heater of the tube, second heater filament is to gnd. The input voltage drops to 6V and the output as well (from 6.3v ...

Edit: Values corrected First stage 5E3 using 6N17B: 5E3-1 6N17B.jpg Thank you. With these values I am not getting the - 0.5V on the grid. Best in mind that the heaters are fed with 5.83V From the regulator output I read 6.3V but as soon as I connect the pin 4 to gnd and the 8 to 6.3V, it drops to 5...

The circuit with the 6N17B tube requires an adjustment of the components for the operating point setting as the values or the characteristic curves are very different to the 12AX7 (7025) tube. The Fender circuit cannot simply be adopted. See for example here: sub-mini 5E3.png Thanks for the tip Man...

I have wired a 6N17B following the Fender Twin schematic and pinout from Frog preamp build doc. From the top 1 (plate) is at 7 then clockwise 8 (heater), 7 (grid) and 6 (cathode) https://guitar.com/wp-content/uploads/2016/05/twin_reverb_ab763_schem.jpg For the heaters I am using a IRF830 as voltage ...

By the end it was just due to a faulty bass pot and a v1a asking for a shielded cable on the grid

CheapPedalCollector wrote: ↑04 Jun 2022, 05:03 Sus to me too, wonder if a plate resistor is way out of spec and massively raising the gain.

I would check the three 100k plate resistors on v1, v2

Would be weird, these resistors are there for over 50yrs

Solved! With the oscilloscope I have isolated the issue to v1b (high input) grid resistor and the jack itself. I have installed a shielded wire grounded only on the side of the jack and soldered 68k resistor to it then to grid. At high volumes and close the amp the squeal comes back so I will instal...

Thanks for the tips. With humbuckers is actually more subtle but it is happening and it is still happening without any instrument plugged, just jack into high input. However i have replaced 2 leaky capacitors and now voltages are more stable but on v1 and v2 there is still oscillation. I have also i...

You shouldn't even have the power tubes in while you have the oscillation. I suggest taking your amp to a qualified technician. I just want to probe the circuit with the oscilloscope. One last try So to recap, in case of oscillation the rules of thumb are: - take out power tubes - look for micropho...

Great! Grounds are checked. ..It could still be Grounds, but... Power supply caps. Is the most likely thing. I don't recommend that you mess around removing parts or testing anything until the Electrolytics in the power supply are checked, and most likely replaced. I'm actually surprised that your ...

Great! Grounds are checked. ..It could still be Grounds, but... Power supply caps. Is the most likely thing. I don't recommend that you mess around removing parts or testing anything until the Electrolytics in the power supply are checked, and most likely replaced. I'm actually surprised that your ...

All mullard caps unsoldered and checked. All of them are fine. Actually someone has been here, probably when I bought it 20yrs ago. 100nf and 680nf caps are Mika and fine. Some pots have been replaced. Checking the pots I have noticed that the two newer had a metal touching the chassis and every pot...

How long ago were the caps changed? Oscillations can be tricky to find. I agree with bmxguitars, it's probably a high freq oscillation you can't hear, you should have no voltage on the grids of any stage. You can't test for leakage by measuring across a cap with another in parallel, lift the leg th...

I don't see how you are getting negative voltages on the grid of any common cathode amplification stage. I think you are instead getting oscillation that your meter is integrating to read as negative. Oscillation above hearing range is common and during some cases, that oscillation can be moved to ...

I have checked if the coupling capacitor between v1 and v2 is leaky by measuring voltages a cross it and bridging it with another cap but no luck on this. I have also tried to replace v1a grid wire with a shielded one and nothing. Right now I am measuring negative voltages (depending on preamp pot) ...

So yes, the issue is located at 1st preamp tube. Weird fact v1a grid gets negative voltage (-5) with preamp pot maxed. Whenever the voltage stays positive there is no oscillation. Can be filtering caps not working properly?