Voltage drop and compression

[Auke Jolman]

Hi all,

A few years back I've build de dual preamp 50W plexi/2204 worked out by Steve. The amp sounds right an nothing out of the ordinary, except...

Recently I've been playing a bit louder than I used to and I noticed guit some compression whilst doing that, especially when engaging the 2204 channel. I can’t say I like that so much.

Today I did some voltage checks. When idle pin 3 of the EL34 powertube reads 386 V. When playing louder the voltage drops to about 310 V.

So the voltage drops about 80 V. Is this what causes the heavy compression that occurs and if so, how can I make sure that does not happen (other than playing less loud  )?

Kind regards,
Auke

[sluckey]

Sounds like your PT is undersized.

[Auke Jolman]

Hi Steve,

Thanks for the reply.

The next thing is how to determent the current draw so I can get a suitable powertransformer.

Can I get the current draw from the datasheet of for example the JJ EL34 (100 mA x 2) and ad the current draw from the preamp tubes (approx. 2 mA x 4) an add them up? Or is that to simpel?

Sincerely,

Auke

[shooter]

I get the current draw from the datasheet

while you're looking, spec out the filaments also

[PRR]

You also want G2 current, which in EL34 is significant.

The dummy thing to do is just buy a PT sold for 50 Watt Marshalls.

> the filaments also

True; but he has tolerable idle voltage, his low-volt happens only when LOUD.

[Auke Jolman]

Thanks for all the replies.

I found this transformer: http://www.tube-town.net/ttstore/Transformer/Power-Transformer/Marshall/TT-Powertransformer-Marshall-Style-45-50-Watt::5991.html

This one is sold for a 50 W Marshall type amp. What puzzles me is that it is only rated for 150 mA and 6 A for the filaments. So the filaments is simply adding the currentdraw of the filaments together, in this case 2x 1,5 A (EL34) and 4x 300 mA preamp tubes, which gives 4,2 A, so that's sufficient. That I get.

But the 2x 100 mA for the EL34's and also taking into account the G2 current (15 mA each?) sums up to 230 mA for the powertubes alone. How can such a transformer be sold to be used in een 50W amp. That I don't get.

What am I missing?

Auke

[sluckey]

This one is sold for a 50 W Marshall type amp.

It will work fine.

Tell us about the transformer that's in the amp right now. Even your idle voltage seems very low. I would expect 450v to 500v B+. Are you using silicon rectifiers?

[tubeswell]

In terms of heaters - Each EL34 draws 1.5A. Each 12AX7 draws 300mA (in 6V mode) (So that's 3.9A), plus (say) 300mA for a 6V lamp (if you're running a 6V Lamp) which bring the heater winding up to 4.2A, and you want a bit of extra capacity for when the Pt gets hot, so say 5A or 6A rating for the heater winding.


The HT winding wants to be rated for 200mA

[Auke Jolman]

I'm using this transformer right now: https://www.tonefactory.nl/power-trafo-104.

I'm using silicon rectifiers indeed.

Still got me puzzled that 150 mA with 350-0-350 CT will do the trick though.

[kagliostro]

Clicking on your link a 404 error appears to me

I think the transformer you have linked is this



correct ?

Franco

[PRR]

> Even your idle voltage seems very low.

Agree. Over 450V.

Rectifiers. How much main filter capacitance? Any deviation from conventional Marshall plan?

Load impedance. Should be around 4K. If you use a wrong OT or tap it wrong, it may be working as 2K. Tube current wants to double. It can't, but will be much higher than expected. (Meanwhile voltage swing and power output are reduced.)

> I'm using this transformer

Ah, yes, 400V DC. And aimed at a self-bias amp which does not have large current swing.

_____________
I agree the FULL output DC demand would be 200mA. But the idle demand more like 100mA. We can't generally run speech/music audio at FULL output steady. Though guitarists may come close. It is not impossible the original Marshall spec was "150mA", and cloners copied the number. But whatever the rating, we know good Marshall-style parts will deliver over 50 Watt all night long.

[sluckey]

Still got me puzzled that 150 mA with 350-0-350 CT will do the trick though.

That's what everybody seems to be selling for a 50W Marshall. I'd trust it to do the job.

Your transformer link does not work. I still suspect your PT is wrong for a 50W Marshall.

[Auke Jolman]

The picture Franco posted is the transformer I'm using right now.

I'm following the Marshall line of filtering. First can 50+50 uF giving 100 uF, then a can 50 uF + 50 uF each for the next 2 nodes, further on each node 47 uF caps on the board. I've used Steve's layout.

How is it that 150 mA does the job fine. Is this because of the HT?

[sluckey]

The picture Franco posted is the transformer I'm using right now.

That's not a good choice for this amp.

[kagliostro]

Here you can see that OT spec are to be suited in compliance with B+ level

if you haven't an OT that match with your B+ (as PRR say) increases the current request (and so the voltage drop)



Franco

[Auke Jolman]

For the OT I have a MM Toneclone O50 JM.

[Auke Jolman]

Hi again,

I've changed the PT for the following transformer 340 V 600 mA without centretap using a bridgerectifier giving me 487 V on pins 3 of the EL34's.

When selecting the 2204 channel and increasing the volume decreases the voltage to about 470 V. But when selecting the Plexi-channel, the voltage drop is much more, somewhere around 440 V.

Is this what can be expected or do I need to search for errors in the amp. I've switch out the powertubes for another pair I had, but the result in voltage drop is about the same.

Auke

[sluckey]

I've changed the PT for the following transformer 340 V 600 mA without centretap using a bridgerectifier giving me 487 V on pins 3 of the EL34's.

Did you change the bias supply also?

I would expect a voltage drop when running this amp at full volume. But I can't say how much. How does the amp sound with that beefy PT?

[Auke Jolman]

Hi Steve,

Yes, I did change the bias supply. The transformer has a dedicated tap for the bias voltage. I changed the 47K resistor to 22K. I set the bias at 36 mA. The original schematic of the amp I plundered show's a dedicated bias supply of 50 V and 50 mA which I used. When I measured the voltage it read 71 V.

The compression that I had with the previous transformer is not there now, so that's ok by me.

To my ears the 2204 channel sounds a brighter, which is ok because I found it to be a bit on the dark side with the previous transformer. The plexi channel sounds fine and I hardly can notice any compression although the voltage drop is there.

[sluckey]

Good deal. I'd say your troubles are over.

[Auke Jolman]

Yes Steve, I like to think so.

The only thing now is that the amp has more hiss and a hint of hum. Can this be caused by the higher voltage?

[sluckey]

Probably.

[Auke Jolman]

Hi all,

I took al look at the choke I used in this build and somehow I had one rated for 50 mA. I know 2x EL34 and the preamp tubes use way more. So could this contribute to the voltage drop at higher volumes?

[pompeiisneaks]

Possibly, The 'marshall style choke' that Doug sells here is 120mA so that seems possible.  (even if it isn't causing sag, you may cause it to fail if too much current rolls through).

~Phil