Poor Man's Tube Gain Make Up Stage

[CJ]

howabout that Watford game,

 

[dirtyhanfri]

Thanks Ian, you're great

The pad is in the correct position as is the phantom power. You can also add a phase change switch here if you want. Jensen has an app note that shows how to do these three at the input:

http://www.jensentransformers.com/as/as016.pdf

Yes I've looked at this jensen app note, just wanted to be sure that would be a good way

The gain control will work where you have it. It is not ideal but in a one tube mic pre there is not much  alternative. You just need to make sure the leads to and from it are as short as possible ans screened. This is a high impedance part of the circuit and is susceptible to interference. It does need to be wired as a pot so a connection to ground is needed from the bottom of the pot.

Ok, thanks for the advice I was thinking in placing the input transformer near the xlr, but maybe it's better to put it near the front panel. And, the pin 1 in the Gain lorlin should act as the bottom of the pot right?

You can have an output pot on the output of a mu follower stage like this but normally only if it is feeding another tube stage. It will not drive a low impedance pot and a following transformer. If you want an output pot so you can drive the tube harder with the gain pot and reduce the output with the output pot then I would recommend using a 5K output pot. The 6922 SRPP/mu follower will drive this to quite high levels producing some nice smooth tube distortion. The output impedance of this pot should be no more than about 1.5K. If you want a transformer balanced output I would recommend using a 10K:10K type.

Ok, I'll get a 5k pot (I have 10k and 47k around, but I'll take a walk to the comps shop.

Thanks, I'll keep this updated...

 

[ruffrecords]

howabout that Watford game,

What about the Norwich City game - we avoided relegation and the club gets £30 miilion in TV rights next year.

Cheers

Ian

 

[dirtyhanfri]

Well, I'm not sure about how to wire the input transformer, I drawn a basic diagram

Is this ok? The primaries are in parallel in order to get 13:1 ratio, but I'm not sure if the secondaries should be too in parallel, or in series, this is the first time I use a input transformer, and my knowledge about transformers is not very wide...

Could somebody check it or give me some advice? Thanks in advance

EDIT: It seems to work; I've got sound going to the tube and at his output, I control the signal going to the tube with the gain switch, I have sound coming from the output pot (10k, was what I've lying around), but not  in the output xlr, I'll check the output transformer wiring....

In advance I can say, at least in my test speaker the "Character" pot sound scratchy as it gets turned (Not connected to ground)

 

[dirtyhanfri]

If you want a transformer balanced output I would recommend using a 10K:10K type.

Just noticed this, my output transformer is 2.4k:2.4k, so I suppose I need to get another transformer...

 

[ruffrecords]

If you want a transformer balanced output I would recommend using a 10K:10K type.

Just noticed this, my output transformer is 2.4k:2.4k, so I suppose I need to get another transformer...

A 2K4:2K4 transformer will be fine as long as you load it with no less than 10K.

Cheers

Ian

 

[dirtyhanfri]

It's alive! ;D

It passes audio, no big hums or noises, now I'm gonna work on the Pad, HPF & Phantom to make a real mic pre.

EDIT: as far as me & a sine sweep in fuzzmeasure can tell there's no difference turning the "character" 10k pot, just a 3db drop in gain

 

[ruffrecords]

It's alive! ;D

It passes audio, no big hums or noises, now I'm gonna work on the Pad, HPF & Phantom to make a real mic pre.

EDIT: as far as me & a sine sweep in fuzzmeasure can tell there's no difference turning the "character" 10k pot, just a 3db drop in gain

It will depend on how hard you drive it. At +4dBu output the distortion will be quite small. Whack it harder so it outputs +15dBu or more and you should start to hear some serious tube sound in the SRPP position.

Cheers

Ian

 

[dirtyhanfri]

You can have an output pot on the output of a mu follower stage like this but normally only if it is feeding another tube stage. It will not drive a low impedance pot and a following transformer. If you want an output pot so you can drive the tube harder with the gain pot and reduce the output with the output pot then I would recommend using a 5K output pot. The 6922 SRPP/mu follower will drive this to quite high levels producing some nice smooth tube distortion. The output impedance of this pot should be no more than about 1.5K. If you want a transformer balanced output I would recommend using a 10K:10K type.

It will depend on how hard you drive it. At +4dBu output the distortion will be quite small. Whack it harder so it outputs +15dBu or more and you should start to hear some serious tube sound in the SRPP position.

This made me think... What about putting a second tube stage after after the pot and before the output transformer?
I think that could give me the gain I'm missing right now, and the "character pot" would have more sense there, where I can get harder into the tube. 5k is fine for the pot before the second tube stage?

And, for the output transformer, 2k4:2k4 is still fine?

 

[ruffrecords]

This made me think... What about putting a second tube stage after after the pot and before the output transformer?
I think that could give me the gain I'm missing right now, and the "character pot" would have more sense there, where I can get harder into the tube. 5k is fine for the pot before the second tube stage?

And, for the output transformer, 2k4:2k4 is still fine?

Yes, a second stage would tend to always work at a higher level and it would be easier to drive it into distortion.

The 5K pot at the output was to ensure a reasonably low output impedance yet not to overload the the tube stage. For the pot between stages you do not need the low impedance output so you can use a much higher pot and ease the load on the stage driving it. Anything up to 100K would be fine. On the other hand, you might want to load the first stage with 5K so as to ensure that it can also add some tube tone as well.

Cheers

Ian

 

[dirtyhanfri]

Anything up to 100K would be fine. On the other hand, you might want to load the first stage with 5K so as to ensure that it can also add some tube tone as well.

I don't fully understand this, what you men by load the first stage?

 

[ruffrecords]

Anything up to 100K would be fine. On the other hand, you might want to load the first stage with 5K so as to ensure that it can also add some tube tone as well.

I don't fully understand this, what you mean by load the first stage?

You were asking about a pot before the second stage. This pot is  driven by the first stage and so it is the load for the first stage.

Cheers

Ian

 

[dirtyhanfri]

Ok, thanks for the explanation.

Well, I got it working, With lots of gain. I'd like to work a bit more on the attenuator before the first tube, it's giving me only -6 db's, I want to get something like -18 in six steps.

To avoid the scratchiness of a pot with dc you could retain the 10K and place a pot (or a switched resistor) with a series cap across it to alter the ac conditions of the circuit. Sounds like quite a fun experiment to me.

I'll try this; If I understand right I must keep the 10k resistor and connect the pot-cap in parallel with the resistor? Wich value and V rating would be fine for the cap?

 

[ruffrecords]

Ok, thanks for the explanation.

Well, I got it working, With lots of gain. I'd like to work a bit more on the attenuator before the first tube, it's giving me only -6 db's, I want to get something like -18 in six steps.

You might be better off with a 20dB pad at the mic transformer input

To avoid the scratchiness of a pot with dc you could retain the 10K and place a pot (or a switched resistor) with a series cap across it to alter the ac conditions of the circuit. Sounds like quite a fun experiment to me.

I'll try this; If I understand right I must keep the 10k resistor and connect the pot-cap in parallel with the resistor? Which value and V rating would be fine for the cap?

That's right. A 250 volts rating cap should be OK. Try a 2.2uF to start with.

Cheers

Ian

 

[dirtyhanfri]

I tried it and the result is no sound  :-\

 

[ruffrecords]

I tried it and the result is no sound  :-\

Are you saying you add the cap and pot and get no sound? How did you connect the cap and pot?

Cheers

Ian

 

[JoshuaUnitt]

Hello Ian,
I know this thread hasn't seen a ton of discussion in a while but I was hoping to get some advice regarding using the PMTubeGainMakeup circuit for a guitar pedal, as I am fairly inexperienced with tube circuit design.

Basically I'm hoping to put together a pedal that features a Fender-derived tone stack, with tube makeup gain, and possibly a choice of balanced or unbalanced out (so it can be used in a pedal chain/before an amp, or as a tube direct box with EQ).

So my question is, how reasonable would it be to drive a passive tone circuit like the standard 3-band Fender amp EQ with a cathode follower, and follow the tone circuit with your mu-follower board? Or would i see more benefit from converting the mu-follower to an SRPP?

 

[ruffrecords]

Basically I'm hoping to put together a pedal that features a Fender-derived tone stack, with tube makeup gain, and possibly a choice of balanced or unbalanced out (so it can be used in a pedal chain/before an amp, or as a tube direct box with EQ).

So my question is, how reasonable would it be to drive a passive tone circuit like the standard 3-band Fender amp EQ with a cathode follower, and follow the tone circuit with your mu-follower board? Or would i see more benefit from converting the mu-follower to an SRPP?

I think that should work. The cathode follower will give a nice low drive impedance to the Fender tone stack and the PMTGMU will provide the necessary make up gain and a nice high input impedance. If you want an unbalanced output then the mu follower will be fine  but it will be very clean. The SRPP will also be fine but it will give a bit more distortion. There are several options if you want a balanced output bit there are solutions with a mu follower and ones with an SRPP.

Cheers

Ian

 

[skeeler87]

Hey Guys!

So I'm building a 2ch pultec eq with the PMEQP1A Mk2 boards and I'm thinking of using this circuit for make up gain. However, I've never worked with tubes before. Do you guys think this would be a good first tube project for someone totally inexperienced with tubes?

Thanks!

 

[ruffrecords]

Hey Guys!

So I'm building a 2ch pultec eq with the PMEQP1A Mk2 boards and I'm thinking of using this circuit for make up gain. However, I've never worked with tubes before. Do you guys think this would be a good first tube project for someone totally inexperienced with tubes?

Thanks!

It is very difficult to advise anyone on the best first tube project. Tubes use potentially lethal voltages so extreme care is needed during construction and test. I helped my nephew build his first tube project  some years ago and after that I felt he was knowledgeable enough to go it alone so finding someone experienced to help you with your first project might be a good idea. Having said that, I am sure there are plenty of groupDIY members who have been through the "first tube project" experience who can give you much better guidance. After all, I built my first tube project back in 1960.

Cheers

Ian