Some bluesmaster PI questions

[greiswig]

I'm still trying to narrow down candidates for the fizz/IM distortion/oscillation that I hear particularly on low notes in OD on my non HRM build. Stock D'Lite plate/cathode pairs, but a Skyliner stack. Part of the trackdown involves voltages...about which I have questions.

1. What is the desired plate voltage on the PI in a bluesmaster? I thought I had heard that 280V was desirable for V3 in the stock D'Lite, but with the trimmer in place I am getting 262 and 258 on V2a and b respectively.

My dropping string looks like this, with V+ measurements over it:

-----------430-------------420--------------316---------------310
300ohm----|----1kohm----|----22kohm----|----2.2kohm----|----150kFET

2. V1a and b are at 190/196, V2a and b are at 196/197 respectively. Those are about right, ain't they?

3. How are you supposed to adjust that trimmer?

[odourboy]

Your voltages look good to me. Setting hte PI trimmer was discussed here:

https://tubeamparchive.com/viewtopic.ph ... sc&start=0

[Fischerman]

Some snips (both from dogears) from a thread mat did a while back:

The PI voltages are different because Mat is using the Bluesmaster PI that Dumble switched to in his later years. Same as in Carlton's amp, among many others. Still his voltages are whacked here. He should have another 40V on his PI pin 6 (Mat had 211vdc here). This PI has lower voltages and when working right sounds amazing.

You can't get 300V with the style PI Mat has. Anywhere in the 240-270V rage is normal. I get about 10V between sides. It is not about the volts but the AC balance...

FWIW, my amp is Skyliner/HRM/BM PI and I have 435vdc on the power tube plates...my PI B+ is 410vdc and my plate voltages are 270/260vdc.

Here is that thread...lots of good stuff in it.

[dogears]

1K is too low on the PI dropping resistor.

The real Dumble BM amp I had here, had 428V on the plates. It had 3.3K going to the PI. I don't know why people insist on using the 1K value in the Dlite layout. It is just not correct if you are building a D-style amp. Especially with 430V from the supply.

The BM amp had the newer 15K/10K string for V1 and V2. 178V on V1 and 200V on V2. PI was about 250V.

Maybe increasing that 1K will help as it will decouple more?

[Structo]

I have a stupid question.
When you guys talk about plate voltages, are you stating the actual voltage say on pin 6 of the tube or are you taking that plate voltage and subtracting the cathode voltage for your number?

So if I feel my voltages are too high on all the 12ax7 tubes will changing the choke resistor down to say 330 ohms drop all the voltages since it feeds the dropping string in series?

My tube terminology isn't the greatest yet.

[greiswig]

Thanks, all.

Scott, I tried a 3.3k in that slot per your suggestion, but the V1 and V2 values all went into the mid 180's. I felt that my clean tone suffered as a result.

I guess the relevant question is which is more important:
a) sticking to the resistor values that HAD used, or
b) obtaining the desired voltages at the plates?

Is it that we know HAD's opinions on a, but not on b?

1K is too low on the PI dropping resistor.

The real Dumble BM amp I had here, had 428V on the plates. It had 3.3K going to the PI. I don't know why people insist on using the 1K value in the Dlite layout. It is just not correct if you are building a D-style amp. Especially with 430V from the supply.

The BM amp had the newer 15K/10K string for V1 and V2. 178V on V1 and 200V on V2. PI was about 250V.

Maybe increasing that 1K will help as it will decouple more?

[dogears]

I would put the 3.3K back in. You can lower the 22K dropping to bring the voltage back... That is the better way IMHO.

Thanks, all.

Scott, I tried a 3.3k in that slot per your suggestion, but the V1 and V2 values all went into the mid 180's. I felt that my clean tone suffered as a result.

I guess the relevant question is which is more important:
a) sticking to the resistor values that HAD used, or
b) obtaining the desired voltages at the plates?

Is it that we know HAD's opinions on a, but not on b?

1K is too low on the PI dropping resistor.

The real Dumble BM amp I had here, had 428V on the plates. It had 3.3K going to the PI. I don't know why people insist on using the 1K value in the Dlite layout. It is just not correct if you are building a D-style amp. Especially with 430V from the supply.

The BM amp had the newer 15K/10K string for V1 and V2. 178V on V1 and 200V on V2. PI was about 250V.

Maybe increasing that 1K will help as it will decouple more?

[Fischerman]

You can then lower that 22k to get your V1/V2 voltages back up...which will also pull your V3 voltages down a tiny bit. In my amp, using a 3k-15k-10k string gets me real close on the voltages (but I'm using a Fender choke and also have a built-in D'lator tapping off the screen supply).

EDIT: Oops...lil late lol.

[dogears]

To answer your question, HAD did alter the value of the PI dropping resistor in many amps. I have never seen it low. I have seen him raise it quite a bit on amps with high B+. I have seen the next resistor lowered to get more voltage on the preamp. 1K is not a good idea IMO since you are not decoupling the stages enough.

I guess the relevant question is which is more important:
a) sticking to the resistor values that HAD used, or
b) obtaining the desired voltages at the plates?

Is it that we know HAD's opinions on a, but not on b?

1K is too low on the PI dropping resistor.

The real Dumble BM amp I had here, had 428V on the plates. It had 3.3K going to the PI. I don't know why people insist on using the 1K value in the Dlite layout. It is just not correct if you are building a D-style amp. Especially with 430V from the supply.

The BM amp had the newer 15K/10K string for V1 and V2. 178V on V1 and 200V on V2. PI was about 250V.

Maybe increasing that 1K will help as it will decouple more?

[Pete]

I have a stupid question.
When you guys talk about plate voltages, are you stating the actual voltage say on pin 6 of the tube or are you taking that plate voltage and subtracting the cathode voltage for your number?

So if I feel my voltages are too high on all the 12ax7 tubes will changing the choke resistor down to say 330 ohms drop all the voltages since it feeds the dropping string in series?

My tube terminology isn't the greatest yet.

PIN 6.
Also I think changing the "choke resistor" while still keeping it in an "acceptable " range will not allow you to effect your voltage much farther down the string. (in other words it is not likely the appropriate place to try to adjust voltage) HTH

[greiswig]

Legend: top row is actual plate voltage, middle is voltage at the drop string, bottom row is the drop string R values

Okay, so here is what I have currently

-------------------------V3(260ish)------V2(200ish)------V1(190ish)
-----------430-------------420--------------316---------------310
300ohm----|----1kohm----|----22kohm----|----2.2kohm----|----150kFET

If I change the string, here is what I would expect

-------------------------V3(245ish)------V2(200ish)------V1(190ish)
-----------430---------------410--------------316---------------310
300ohm----|----3.3kohm----|----18kohm----|----2.2kohm----|----150kFET

So I have reduced the voltage to the PI without doing the same elsewhere. Now, why would I want to do that?

[Structo]

So what are the advantages to dropping the choke resistor value?
Does it tighten up the bottom end as Scott suggests?

Sorry for the intrusion Greiswig but I'm trying to understand the same thing as you for the dropping string.
Except on my stock values I am seeing over 345v on the PI plates.

I'm going to throw in a 330 ohm choke and see what that gives me.

[Normster]

So what are the advantages to dropping the choke resistor value?
Does it tighten up the bottom end as Scott suggests?

Sorry for the intrusion Greiswig but I'm trying to understand the same thing as you for the dropping string.
Except on my stock values I am seeing over 345v on the PI plates.

I'm going to throw in a 330 ohm choke and see what that gives me.

Going from 500R to 330R won't change your PI voltage much. If you're getting 345VDC on the PI plates, there's something wrong (either with the tube or the circuit). What are your cathode and grid voltages? It almost sounds like you've got the wrong tail resistor value on your PI, or maybe it's not grounded properly. Does your presence control work properly?

[Structo]

Thanks Norm, I will double check those.
I'm using the JJ ECC83S for the PI tube, it is the one that came with the kit.
Which one is the tail resistor? The 820 ohm?

I'll check the resistor values again but pretty sure they are what the final layout shows from BN.

Yes, the presence works and I rewired it to the way Scott told me.
With legs 1 & 2 jumpered and the 1uf tant cap from leg 1 to the buss.
And with the 390 ohm resistor from leg 3 to the buss.
Wire lead from the 6K2 resistor to leg 3.

I mean the amp sounds pretty darn good.
I was just a bit surprised when I measured the voltages.
I'll pull the chassis again and record the values so I don't have to rely on memory for those.

[greiswig]

Sorry, I may have to ask this question again as it seems to have been lost:

Legend: top row is actual plate voltage, middle is voltage at the drop string, bottom row is the drop string R values

Okay, so here is what I have currently

-------------------------V3(260ish)------V2(200ish)------V1(190ish)
-----------430-------------420--------------316---------------310
300ohm----|----1kohm----|----22kohm----|----2.2kohm----|----150kFET

If I change the string, here is what I would expect

-------------------------V3(245ish)------V2(200ish)------V1(190ish)
-----------430---------------410--------------316---------------310
300ohm----|----3.3kohm----|----18kohm----|----2.2kohm----|----150kFET

So I have reduced the voltage to the PI without doing the same elsewhere. Now, why would I want to do that?