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RE: GRAAF GM 200














I own a Graaf GM200 and it is both an absolutely stunning sounding and looking amp. Just bought a second one (along with a matching GM 13.5B II preamp and GM70 phono preamp - a bit like finding hen's teeth to locate these two) so that I can bi-amp a pair of Wilson Benesch Bishop speakers with them. As you can see, the bug has bitten very hard with this one...

As far as the tubes that can be used with this amp, either the PL504 or the 27GB5 will work (I've successfully used both). The PL500 has the same characteristics except for anode dissipation - 12W max for the PL500 rather than 16W for the two mentioned.

http://tdsl.duncanamps.com/pdf/pl504.pdf
http://tdsl.duncanamps.com/pdf/pl500.pdf

An important point to make about the tubes - all PL504/27GB5 tubes are definitely not equal. This amp will make you tear your hair out if you don't use QUALITY tubes - they will blow time and again, each time requiring you to open up the amp to unsolder and replace a blown 1 Ohm resistor under the faulty tube. 'Quality tubes' means using either tubes made in Europe or Japan.

There are also quite a variety of PL504/27GB5 tube designs (I have come across at least 6 or 7) and the tubes used should be well matched so they should all be of the same design (not necessarily same manufacturer, as the same manufacturer sometimes uses different designs and vice versa). Fortunately the Graaf factory will be able to supply you with a matched set and can service the amp for you - just remember that they are in Italy and life seems to work at a pace there that is about 1/8 of that in the US... I also have a substantial collection of these tubes, some with the required 32+ of the same design, others with less, so happy to discuss tube swapping with interested GM200 owners.

The literature accompanying the amp specifies setting a bias current on each channel equivalent to a voltage measured between the channel's test point and ground of 75mV but the factory now recommends that this be reduced to 65mV to extend tube life and to allow for slightly higher mismatch between tubes. This obviously also reduces the amount of power that the amp delivers in pure class A mode before it transitions to AB.

The offset voltage on each channel should be reduced to as close to 0mV as possible between the + and - speaker posts (less than 2mV is recommended). The amp has a circuit that constantly measures this offset voltage and will switch the amp off if it goes out of spec to protect the speakers.

When replacing the tubes I personally turn the bias pots completely down so that the current through the tubes is 0 (bias voltage is 0). Note here that the left and right channel bias pots turn in different directions (clockwise and anti-clockwise) to get down to 0 - I have a note somewhere that tells me which direction for which channel but I am sitting in London while writing this, so don't have this with me. I slowly turn the pot the appropriate direction until the bias current starts to creep up and then check the offset voltage and adjust the offset pot if necessary to bring this down to 0. I then turn up the bias further until 65mV is reached, making sure along the way that the offset stays at 0 (or close to). I then check the voltage across the anode resistor on each tube to check how well they are matched. They should all read around 15.5mV plus or minus 2.5mV. Any outside this range should be replaced (this can usually be done without resetting bias to 0 if not too many are way out). Bias is then readjusted, offset voltage readjusted, and the anode currents checked again. Note that the anode current in the front 8 tubes on each channel is slightly higher than the anode current on the rear 8 tubes. Someone technical on this forum may be able to tell me why Mr. Mariani has designed the amp this way - I am guessing so that tubes with higher anode current readings can be placed in the back group and those with lower readings can be placed in the front group to get the closest possible match of current through all tubes, but this is just a guess and I might well be wrong.

I have attached a picture on the amp. I also have some schematics that give some but not all details - these are for one of the two potted blocks in each channel of the amp - the single-ended/balanced input module (there is another potted module on the driver board (a coupling capacitor and resistor network)). I have included a circuit that will replace the input block with a passive single-ended input (does away with one input tube).

I do have the service manual that gives details of the full circuit, wiring diagram to change line input voltage to 220V/110V/etc. Feel free to contact me if you need this - haven't worked out a way to upload this .PDF file.


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