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    Building, troubleshooting and testing of these amplifiers should only be
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Graaf GM20 mods - help!

Am30

Based on the GM20 design.
An elapsed time counter was mounted.
No guesswork any more.
 

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Ok, great idea. I saw that already in your link.

And what about the 30 W Power Output. Do you run these with 300 mA bias current
as was originally incorrect given on the PCB of the Graaf GM20?

Written was there near the bias measurement point 0,30 mV which was much to high
and in reality should be between 0,19 - 0,22 mV.

Günter
 
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Bias

I adjust bias for 240mA. It's stable and with the cooling it's not running hot. Tube life is acceptable. I haven't replaced any so far, but less than 1000 hrs.
As mentioned, the first I built as "hard wired" and with counter, the tubes were replaced after 3000 hrs. No failures.
 
Ok, I could adjust the bias on my clones as well up to 250 mA.

The builder I bought them from advised 200-250 mA max and I'm staying at 200 ma
for longest possible tube durability.

However I'm wondering if 240 mA bias will really give 30 W power output and at which impedance by the way?

I would assume that it is somewhat less.

Günter
 
Hi,

Ok when you have measured it like that.

When you say impedance on the Tannoy drops below 3 Ohm which is not good I would
say it depends on where and I mean at which frequency that is.

I run mine with Silberstatic No.1 fullrange electrostatic speakers and they have an
impedance minimum of below 1 Ohm at 16 kHz!

You have to take into account when looking at impedance minima that 70% of all the power
a speaker requires from the power amp is in the bass and midrange and that means
up to around 3 kHz I would say.

The transformers in my electrostatics are designed "tube friendly". Until 3.8 kHz the speakers
have an impedance of 16 Ohm and higher and it remains 8 Ohm and higher till 6.5 khz.
4 Ohm is reached at 10 khz and impedance drops down further to the minimum at 16 kHz.

And guess what the OTL's have no problem with that!
That is because there is only little current required at such high frequencies.

Apart from that the OTL's have as opposed to transistor amps more power at 16 Ohm than at 8 Ohm
which is perfect for electrostatic speakers with their higher impedance in the bass and midrange.

So I don't worry about the low impedance as the amps are voltage stable which is what the electrostatics really need.


Günter
 
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Taking the heat away

Taking the heat away:
Here is an effective way of reducing the heat. A wooden block with blowers as base plate for the amplifier. The blowers are absolutely quiet. It will reduce the chassis temperature of a GM20 by 20°C.


Hi Hans,

Referring to your thread from 2009 #23 "Taking the heat away" I would like to get somewhat more detail information on the blowers you are using.

For chilling the tubes I'm using right now passive cooling by means of Pearl tube coolers, see here:

PCF-175: 1.75" Dia. Power Tube Cooler

These devices dissipate warmth away from the glass of the 6C33 tubes working like heatsinks and as such can have a certain positive effect on lifespan of the tubes but they cannot
really lower temperature significantly like an active cooling with blowers as you do.

What I can see on your pictures is that they work with 12V DC and as I see only two red/black wires from each blower connected parallel to the power switch I assume that they work with a fixed
rotational speed ( not adjustable ), right?

What brand are these blowers and could you provide their part nr. please?

I would be especially curious at which rotational speed in rpm you run them and how silent they are ( noise figure ? ) .

Would be great if you could answer this as I would like to try that kind of cooling as well in the near future.


Thank you very much.

Günter
 
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Cooling blowers

Hello Hans,

Thank you very much for plenty information on your chilling solution with the blowers. I appreciate that a lot!

Having looked up the specs of these fans I found that these are running at 500 rpm with a very low noise figure of only 7.5 dBA which is indeed extremely silent.

According to the Scythe website this 500 rpm 12SL version is discontinued however I have been able to find a source where I can still buy them at a very low price.

Only question remaining for me is why you run them with 11 rather than nominal 12V DC which results of course in slightly lower rpm.

So what is your concern or how did you find out to run them at 11V when you say "Adjusted for proper cooling. About 11V."?


Thank you once again.

Günter
 
Hi there, new to this forum and I am not so much a DIY-er but I own some Graaf equipment (Modena, 13.5B2 and GM70) that I am very enthusiastic about. I follow everything there is to know about this equipment, especially the Modena, so that I treat them well and they will stay with me until the length of time. I am very happy with this audio gear.
 
Hi DutchMediator,

Regarding your nickname I suppose that you are living in the Netherlands.

If this is true then maybe this website is interesting for you regarding your Modena's which are actually two GM20's in Mono configuration assumed that you don't know them already:

https://www.analogueaudio.nl/graaf-gm20-modificatie.shtml

So they do a mod allowing to adjust Bias and DC offset from outside i.e. not requiring to open the bottom cover in order to do this, may be a worthwhile modification for you as this makes adjustment very comfortable.

Reading what they say there I agree with them for the most part but not when they say that the well-thought-out schematic of the GM20 cannot be improved which in my opinion simply is not true as I said here already under #35.

Regards,
Günter
 
Did anone tried to upgrade the interstage capacitors (C1, C2) with sth like Mundorf Supreme Silver/Gold/Oil 1,00 uF 1200VDC

mcap_supreme_evo_silvergold(1).jpg


(please not the pic shows 4.7uF)

or, if the Mundrf doesn't fit, sth like

Mundorf M-CAP Supreme Classic Silver/Gold 1,00uF 1000 VDC

ZFW-5072880.jpg


or Jantzen Audio Superior Z-Cap 1,00uF 800VDC ?

001-0534-eb70.jpg