New 45 build has weird voltages - need guidance please!

[page361]

Well so far my troubleshooting seems to have narrowed the drop in DC voltage to the phase inverter - double checked the under board wiring and noticed the two blue wires on V3 were indeed backwards - fixed that thinking I may be OK...fired it up...checked voltages again...still the same severe drop of B+.
One thing I haven't done yet is to swap OT primaries at V4 and V5 - but before I do I was wondering if that could make the difference in this instance?

[OnTheFritz]

If the primaries are reversed you'll get obnoxious motor-boating, so that won't be the problem.
I've had problems in the past with the underboard buss wires looking fine, but being pinched and shorted via the stand-off studs when the board is securely cranked down. I'm late to the party, but hoping that's been ruled out.

[neikeel]

You will have noticed I have not referred to reversed primaries (this will test completely normally without valves in the amp) as this only gives you positive feedback when you have the valves in and powered up.

If the problem was on the board with a link you would have not got the power drop corrected when you disconnected the OT centre tap (remember you have no tubes in so the other primary wires are effectively open circuit on pin 3 of the output sockets).
Similarly the choke.

[page361]

Thanks ...so no need to touch the primaries then. The board was lifted off the standoff's and I confirmed that the buss wires were not being pinched - so all is OK there. Again the only issue I found were the V3 blue wires reversed - fixed that quickly.

[neikeel]

Thanks ...so no need to touch the primaries then. The board was lifted off the standoff's and I confirmed that the buss wires were not being pinched - so all is OK there. Again the only issue I found were the V3 blue wires reversed - fixed that quickly.

Switched plate resistors is a minor point and would not affect your issue, would just mean that the asymmetry in your PI would affect different output tubes.

If you are sure your primary and secondary wires are labelled correctly. Did you 'ring out' the primary wires with your meter?

[page361]

Yeah pretty sure - I spoke to ChrisM at length yesterday morning to make sure I didn't mix something up with the OT. But I suppose there's always the chance that the OT drawing provided me does not match the OT''s actualy wiring color code. Gosh I think that may be a stretch though.

[axeman]

Your 250pf at tone stack is hooked up wrong!

[neikeel]

Your 250pf at tone stack is hooked up wrong!

- yes it is (well spotted) but that is not the cause of his voltage being dragged down when everything is connected in the HT line except when the OT centre tap is hooked up with no tubes in.

- need some more info!!!

Can you confirm the above tests?

[axeman]

Post close ups of the filter caps.

[page361]

Here ya go axeman - I'm using the traditional JTM45 choke wiring scheme by having one of the choke wires go directly to the 8.2K turret, then to a 1K3W jumper then to V4 pin6 which goes to V5 pin 6. Finally a separate lead attached between the 32uf lug closest to PT and underside of the 8.2K turret closest to V4.

Hopefully I can explain the symptoms so it makes sense...firstly, I have triple checked the under board wiring and all is correct. When everything is wired up, and GZ tube a V1-V3 tubes are installed (no power tubes) I get 420vdc at the standby switch and GZ. AC at heater wires read perfect. But when standby is turned to ON the vdc drops at GZ and switch down to 70 volts, and the voltage readings at the preamp sockets are the same as per the earlier post in this thread. LOW!

Next, when I lift the 8.2K (side closest to pots) the vdc stays at 420vdc with standby ON - no drop. But when I put the 8.K back and lift the 10K, the voltage at the standby switch and GZ basically drifts up and up rather then settle in at the expected voltage. I can best describe drift as the voltage slowly continuing to climb and then surpassing the expected level.

Also in my troubleshooting, I removed the connection back to the 8.2K from the 32x32uf and the then the 16x16uf can to see if they might be the culprit. ...it did not - same wacky voltages.

Since then I put everything back together and lifted the coupling caps...tested voltages again and got the same "drift" in vdc at stby switch and GZ and the way low readings at V1 - V3.

[neikeel]

When everything is wired up, and GZ tube a V1-V3 tubes are installed (no power tubes) I get 420vdc at the standby switch and GZ. AC at heater wires read perfect. But when standby is turned to ON the vdc drops at GZ and switch down to 70 volts, and the voltage readings at the preamp sockets are the same as per the earlier post in this thread. LOW!

We were going through this step by step and you said that the voltages only dropped when you unhooked the PT - is that not in fact the case

Next, when I lift the 8.2K (side closest to pots) the vdc stays at 420vdc with standby ON - no drop. But when I put the 8.K back and lift the 10K, the voltage at the standby switch and GZ basically drifts up and up rather then settle in at the expected voltage. I can best describe drift as the voltage slowly continuing to climb and then surpassing the expected level.

First time you mentioned this

Also in my troubleshooting, I removed the connection back to the 8.2K from the 32x32uf and the then the 16x16uf can to see if they might be the culprit. ...it did not - same wacky voltages.

Good

Since then I put everything back together and lifted the coupling caps...tested voltages again and got the same "drift" in vdc at stby switch and GZ and the way low readings at V1 - V3.

In that case correct your tone stack wiring, remove the preamp tubes and see what voltages you get..........

[axeman]

Is this correct?

[page361]

Is this correct?

Yeah - I triple check all under board wiring including buss wiring - all good

[page361]

Hey Neil - I discovered the issue of the vdc dropping at the GZ and STDBY switch when STDBY is ON since my last update - same with lifting the 8.2K and 10K...

[page361]

Lifted coupling caps as well as 100k and 82K resistors. Then measured voltage with GZ and preamp tubes installed. Voltage at STDBY switch and GZ when STDBY is in ON position read 84vdc (as opposed to 420vdc when standby is in OFF position).

Most notable is that this time there is no slow "drifting" of vdc up and up as before.

Finally the voltage at V4 & V5 pin 5 measured -94 VDC. Measured pins at V1-V3 and they are basically same as before - VERY LOW.