Hafler T2 gut shots, schematics: mid/treble boost mods?

[ryanpg]

Hi I just got a Hafler T2 guitar preamp. I've taken some gut shots and attached a schematic.

[ryanpg]

Continuing on with pics.

[ryanpg]

The photo below is not of my unit. This one appears to have the transformer still in place and no push-pull gain.

And finally the schematic for version 4 (the photos of mine are V3)

[ryanpg]

Deleted.

[ryanpg]

It appears my preamp (as seen in the pictures) deviates from the schematic in many ways:

- The coupling/output transformer after the tube stage has been replaced
with a couple resistors.
- R60 has been replaced with a capacitor
- R61 is removed
- R23 is removed
- R24 has been changed from 1K to 220k
- R7 has been changed from 150k to 470k Ohms

I would love to get a bit more midrange and brightness out of it.

- R24 was 1k across the secondary of the transformer, now 220k Ohms 5% - no idea why the change other than to decrease signal to ground thus increasing gain/volume? (EDIT: this is wrong, R24 was changed to 220K but never "was 1K across the secondary")

- R60 which was a 10K resistor, has been replaced with a .0047k 400v resistor - no idea why the change other than to allow more signal to the op-amp? (EDIT: the replacement is actually the coupling cap)

- R7 was a 150K resistor, now a 470k Ohms 5% - does this increase in resistance result in less signal drained to ground, thus increasing the output of T1B? Or is it more to adjust the filter created by R7 and C13?

- R6 was a 300K resistor, now a capacitor - this seems to be a part of the process for replacing the transformer? I no idea why and what effect it may have. (EDIT: this is the plate resistor, a cap was wired in parallel to drain some high end. This hid the resistor from view.)

- R61 now deleted was a 4.7k to ground - removed to increase signal to the LF347N?

- R23 now deleted was a 1M - was this a negative feedback loop? Looks like it went from the output of the transformer back into the input of T2A and T2B. Removing the NFB allows for more distortion and gain from the tubes, but at a cost of bandwidth correct? Was this necessary because the transformer is gone?

If I'm right, the changes to R61 and R23 are about increasing gain/volume.

That's fine with me. I'm really unclear about the changes related to the transformer though, I found this on another amp garage thread:

"[the transformer] is used as plate load for T2B and as a decoupling stage to low voltage following stages. R23 is a local feedback loop. Replace TT2DOT with a 100k to supply and a 47n 630v to R24. If you noticed too much bass, use a 22n, if not enought bass, try 100n." and "If you want to try to "mesallize" the sound, try a small cap (100 to 470pF) across the 100k."

Sounds very precise advice, but I really don't understand exactly what's being suggested and how much of the above has actually already been done.

[teemuk]

- R24 was 1k across the secondary of the trasnformer, now 220k Ohms 5% - no idea why the change other than to decrease signal to ground thus increasing gain/volume?

Uh, I'd figure the gain stage wouldn't be too happy with a 1K load. See, the transformer can reflect 1K load as much higher impedance to the plate. Without transformer the tube needs to be connected to much higher load resistance, otherwise the headroom of the stage will be severily limited, not to mention 1K load drawing plenty of current and dissipating plenty of power. For a generic tube stage 1K is about 100 times too low load resistance.

- R60 which was a 10K resistor, has been replaced with a .0047k 400v resistor - no idea why the change other than to allow more signal to the op-amp?

Seems like that.

- R7 was a 150K resistor, now a 470k Ohms 5% - does this increase in resistance result in less signal drained to ground, thus increasing the output of T1B?

T1B output likely remains the same. BUT R7 forms a resistive voltage divider with R8. With 470K/470K division the voltage in the interjoining node drops to about half of that of "input" voltage. With the stock 150K it drops less. The mod is about reducing input signal amplitude at T2A grid. Probably to tame down excessive overdrive of that stage.

Or is it more to adjust the filter created by R7 and C13?

Probably not but you are right that the modification affects corner frequencies of this filter too.

On that note, the RC filters are actually R6+R7 & C31 and C13 & R7+R8.

- R6 was a 300K resistor, now a capacitor - this seems to be a part of the process for replacing the transformer? I no idea why and what effect it may have.

I would have to see the entire diagram instead of a description of it but replacing plate load resistor with a capacitor (and no other mods) will simply render the entire gain stage non-functional. The mod as you describe it doesn't make any sense.

[teemuk]

- R61 now deleted was a 4.7k to ground - removed to increase signal to the LF347N?

Seems so. This was another voltage divider with R60.

- R23 now deleted was a 1M - was this a negative feedback loop? Looks like it went from the output of the transformer back into the input of T2A and T2B. Removing the NFB allows for more distortion and gain from the tubes, but at a cost of bandwidth correct?

Negative feedback corrects distortion and can increase overall cleanness as well as headroom of the stage but at the same time it makes transition to clipping distortion more abrupt. Sometimes it extends bandwidth. In this type of design the bandwidth limit is probably of lesser importance because the circuit aims to it all over the place.

In essence, both removing the feedback and tranformer coupled loading will have drastic impacts on how the stage behaves, especially when overdriven.

Was this necessary because the transformer is gone?

No. Negative feedback can be applied even without a transformer.

[teemuk]

Replace TT2DOT with a 100k to supply and a [sic, this really should read "add"] 47n 630v to R24. If you noticed too much bass, use a 22n, if not enought bass, try 100n." and "If you want to try to "mesallize" the sound, try a small cap (100 to 470pF) across the 100k."

Sounds very precise advice, but I really don't understand exactly what's being suggested and how much of the above has actually already been done.

It is a very simple task to convert a transformer loaded and coupled circuit to a (resistively) plate loaded and capacitively coupled circuit. (The given advise on the other hand is somewhat confusing). But still if you struggle with understanding what this advice and the details of it refer to then, IMO, you have no business inside that preamp.

Admitting that you struggle to understand this is great and shows a strenght of character but unfortunately it also casts a doubt on how much trust one can put to information about the circuit you give.

Why? Because understanding what plate and transformer loaded circuits are and how they differ is basically a key part in 101 of audio tube circuits. Struggling with understandingf the most basic and elementary things cast a shadow of doubt to any information you provide and basically puts you into a position where you likely shouldn't be modifying tube circuits at all because you evidently lack the elementary knowhow to do it.

This means a greatly increased risk to do more damage than good.

[ryanpg]

Teemuk, thanks for the detailed replies and instruction! I appreciate you going through most of my questions and confirming/correcting each.

Struggling with understandingf the most basic and elementary things cast a shadow of doubt to any information you provide and basically puts you into a position where you likely shouldn't be modifying tube circuits at all because you evidently lack the elementary knowhow to do it.

Point taken. Scolding and shooing away novices such as myself seems the appropriate default response when dangerous voltages are present.

[ryanpg]

Deleted.

[ryanpg]

Aha! The mystery of R6 and R60 has been solved!

R6 - Hiding under the 0.001μF cap is a little 300k Ohms resistor!

R60 - what I thought was a .0047k resistor, is actually a capacitor! So R60 IS my coupling capacitor, and R6 IS a plate load resistor in parallel with a cap.

[ryanpg]

To summarize my understanding of the transformer conversion:

A 100k plate resistor has been added across the primary where the transformer was.

C60 .0047μF 400v (which was R60) is now a coupling capacitor. It also acts as a filter (smaller value, less bass frequency).

1M attenuating resistor added following plate.

[roberto]

Hi, I've your PM asking my help, but cannot read all that.
Try to resume what you need, or nobody will reply.

[ryanpg]

Hi, I've your PM asking my help, but cannot read all that.
Try to resume what you need, or nobody will reply.

Hi roberto, My apologies. I'll try to be concise.

There's a real lack of high-end/treble, it's very muddy. I'm not sure where I'm loosing high-mids and treble.

[ryanpg]

I've settled on putting a 0.001μF cap across R7 for now. It allows more highs through. With presence around 8 and tone around 3-4 it's fairly bright.

I also put a 12AT7 in T1 just to mellow out the distortion.

Things I've tried but reverted:

• - A 0.022μF cap parallel with that 1M resistor, I get some brightness back. Too much volume. Signal gets through even when Channel B gain and level knobs are all the way down.
  - Removed the 0.001μF cap parallel to R6, the plate resistor of T1B. It did restore more treble, but made the distortion too "fizzy."
  - Lifting C31 had little impact, I think it only filters VERY high frequencies.
  - Lifting C32 same minimal effect as lifting C31, very high frequencies are being bled off.
  - C9 to .047μF - a bit more bass with a .047μF but mids/highs are the issue with this pre.

If anyone has other ideas, I'd love to hear.

Thanks.