HRM Bluesmaster 50 PI trimmer help

[akabek]

I recently assembled a Ceriatone HRM Bluesmaster 50. It is up and running, quiet and all the controls are working. What I am having a problem with is adjusting the PI trimmer.

This is the process I am following to adjust the PI trimmer:
Inject a 1khz signal into the return jack of the loop and adjust the level of the signal so it measures about 30VAC on pin 3 (plate) of your power tubes. To get the 30 volts I adjust the signal generator input to 650mA to get 30VAC at pin 3 or the power tube.

Then using a DMM I connect the black probe to chassis (ground) and measure the VAC of pin 1 & 6 of the PI tube v3. At pin1 I get 4VAC and pin6 I get 3.7VAC. Adjusting the trimmer only shows a 0.02VAC difference on either pin.

I pulled all the tubes and checked them on my Start 9-66 tube tester. Turns out all the tubes were good except the one I had in v3 for PI was was borderline faulty on one side.

What I have done:
Replace the v3 tube with a balanced tube
I checked the PI trimmer with the DMM and I get appropriate resistance readings moving the wiper so it seems to be working.

I know there are various ways to set the PI trimmer but will the method I used work? Maybe I have a cold solder joint somewhere? I have attached the schematic.

[ijedouglas]

I prefer to measure at the power tube cathodes so you take into account the PI as well as the power tubes. You will still need to set it by ear thereafter as the OT introduces further imbalances.

I use this method described by Gil; https://ampgarage.com/forum/viewtopic.p ... 78#p216778

[akabek]

Thanks Ian.

If the method I outlined is one correct way to do it but my readings are not correct I need to fix the problem. I am indifferent to which method I use. I just want to get in range so I can fine tune by ear.

[ijedouglas]

Thanks Ian.

If the method I outlined is one correct way to do it but my readings are not correct I need to fix the problem. I am indifferent to which method I use. I just want to get in range so I can fine tune by ear.

I use the method Gil describes and have had no issues. Measuring at the PI tube will only tell you if the PI is balanced but does not take into account the power tubes and OT. Try the method Gil describes and see what it gives you?

The trimmer should give you more adjustment of those voltages though.

[martin manning]

A bit further down in the thread Ian linked I added a few comments that might be helpful: https://ampgarage.com/forum/viewtopic.p ... 89#p416389
I agree measuring at the power tube cathodes is the better option.

[akabek]

I gave the Ampeg procedure a try.

I set the power tube bias to 34mV. Both tubes were showing 34mV on the DMM.

I injected a 400Hz signal at 1.5 Vrms into the high input jack

I adjusted the master volume for an output of 18Vrms at the 8 ohm dummy load

Using a DMM I connected the black probe to the chassis for ground and measured the DCV on pin 1 of each power tube. Using the PI trimmer I see a range between 105 - 114 mV. I cannot get down to 0Mv.

[martin manning]

You need to connect the DMM from one cathode pin to the other, and read DC mV. What are you setting to 34V? Do you mean 34 mV?

[erwin_ve]

I gave the Ampeg procedure a try.

I set the power tube bias to 34v. Both tubes were showing 34v on the DMM.

I injected a 400Hz signal at 1.5 Vrms into the high input jack

I adjusted the master volume for an output of 18Vrms at the 8 ohm dummy load

Using a DMM I connected the black probe to the chassis for ground and measured the DCV on pin 1 of each power tube. Using the PI trimmer I see a range between 105 - 114 mV. I cannot get down to 0Mv.

I think you're mixing up bias and PI balance.
If you're measuring 105mV on the cathode you burn your tubes. Set the bias to a appropriate level, rough guess would be around 36-40mV on the cathode and then proceed with the procedure Martin refers to.

[akabek]

When I connect the DMM probe from V4 pin8 and the second probe to V5 pin 8 I can now zero it. I assume from here I just need to do slight adjustment by ear to get it slightly out of phase? Would that result in a + or - voltage reading on the meter?

Does this procedure need to be redone whenever a tube a replaced?

[erwin_ve]

When I connect the DMM probe from V4 pin8 and the second probe to V5 pin 8 I can now zero it. I assume from here I just need to do slight adjustment by ear to get it slightly out of phase? Would that result in a + or - voltage reading on the meter?

Does this procedure need to be redone whenever a tube a replaced?

First step is to make sure you don't burn your tubes, what is the B+ voltage and what is the cathode to earth voltage?(bias voltage in mV is same as mA, due to 1Ohm resistor, Ohm Law)

[akabek]

You need to connect the DMM from one cathode pin to the other, and read DC mV. What are you setting to 34V? Do you mean 34 mV?

Typo on my part. I corrected it to mV. I set the power tube bias to 34mV. Should I bump it up around 36-40mV?

[martin manning]

Measure the plate voltage Vbplus, and then assuming 30W 6L6 power tubes, set bias at 0.65*30W/Vbplus. So for example if Vbplus is 450V, se bias to 0.65*30/450 = 0.0433A, so with 1R current sense resistors you need 43.3 mV.

I recommend injecting the signal at the power amp input.

[akabek]

I am learning as we go so I will spell things out so I have a reference when I need to come back to this.

I have moved the signal cable from the front high input jack to the loop input jack on the rear. The layout diagram shows power amp in.

I injected 400Hz at 1.5Vrms which gave me a plate voltage of 420VDC on V4 and V5 pin3.

0.65*30/420 = 0.0464A, so with a 1R current sense resistor I would set the bias to 46.4 mV.

[erwin_ve]

I am learning as we go so I will spell things out so I have a reference when I need to come back to this.

I have moved the signal cable from the front high input jack to the loop input jack on the rear. The layout diagram shows power amp in.

I injected 400Hz at 1.5Vrms which gave me a plate voltage of 420VDC on V4 and V5 pin3.

0.65*30/420 = 0.0464A, so with a 1R current sense resistor I would set the bias to 46.4 mV.

Setting the bias is typically done when idle. No signal injected. Pin 6 is after the choke Pin 3 is at the plate, you can also use the filter board point where it is marked "OT". The B+ voltage (pin3 or Ot) should be around 460v.

[akabek]

With no signal injected measuring V4 and V5 pin3 I get 456V.

0.65*30/456 = 0.04276A, so with a 1R current sense resistor I would set the bias to 42.7 mV.