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In Reply to: RE: GRAAF GM 200 posted by rhinohifi714 on January 31, 2012 at 19:56:37
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.
Follow Ups:
Hello Mark,
I need a little of your knowledge about Graaf GM200 if it's possible, I find, on power board a little part who became very Hot( not very far of R28 , a name part on board is " MV1" ), I dont find on schematic, this reference neither his value..?
I supose this part is CTP but could you give me value and reference of this MV1 part, and perhaps you could send me better schematic information with technical diagram.
I tried to contact info@graaf.it but nobody answer even by phone...unfortunately.
Could you help me in checking on your GM200's?
(I tried to upload a pics of the part but that's don't work...if you have personal email address i can send you the pics of this piece)
Many Thanks
Best & Kind regards
Fred
I am guessing that what you are talking about is what is called an MOV (Metal Oxide Varistor). It is a component that acts like a high-value resistor until a certain voltage across it is reached, at which point it becomes a low-value resistor. Until the voltage across it rises to the cutoff point, it will dissipate quite a bit of power and it is normal for it to get very hot. Once the cutoff point has been reached (when it becomes a low value resistor) the dissipation will be significantly reduced and it should cool down significantly.
It is usually used to prevent current surges in an amplifier when the power supply is ramping up to its working voltage. Once the PS working voltage is reached it essentially switches itself out of the circuit as its resistance is reduced. I would not be too concerned about this component getting hot unless there is some other fault present or it is looking like it is burnt out.
Here is an article on them:
http://www.cliftonlaboratories.com/metal_oxide_varistor_%28mov%29.htm
Of more concern in the GM200 are some pretty large wire-wound resistors in the power supply that are about 2-3" long. There are two of them, almost immediately after the rectifier bridges, that get EXTREMELY hot if there is a fault in the amplifier (normally they are not used but plugging the amp into an incorrect line voltage e.g. 220V when the amp is set for 110V, would be such a case or if there is a short in a power tube). As they were mounted directly onto the PCB in early amps, there have been rare occasions where they have caused the PC board on some GM200 amps to catch fire! The solution to this (that can be seen on the later constructed GM200s) is that they are installed with long leads that lift them off the board by about 1.5-2" so that they can dissipate the heat properly and not heat up the PCB. If you find your amp has these two resistors mounted directly on the PCB you should probably install two exact replacement resistors that are mounted above the board on their leads.
Hope this helps...
Kind regards,
Mark.
Hello Mark,
Many thanks for your explanation,
that's really nice to find people like you available to share their passion & knowledge to help us.
My Graaf GM200 model (is Balanced) appear to be the second version with the right resistance modification on the power supply...
(with the trick to lift them off the board to dissipate the heat away from PCB) that is pretty a Good News! the Latest revision.
Manifestly the 4xEFL200 failed & need to be change to test as well the tension...
I'm gonna change, as you advice me, these two resistors next to rectifier bridge for exact replacement before change this MOV.(thermistance) to check what happens?
I still investigate and let you know when it's done in hoping there will be no other amp fault or problematic issue.
(i ever changed many 1ohm blown resistor under the PL504... i changed the 32 Output tubes for same ones to keep them matching together)
Anyway, if you know where it's possible to find an other 32xPL504 Good quality set, i Still interested!
(cause in France it's complicate to find large matched quantities from Good Brand & same construction of these old TSF tubes!)
Mr Marianni is unreachable since 2 years, i tried by many way to contact him, even since the launch of the new GM400, maybe already out of business..?
I saw you had 2 xGM200, i Dream to find an other one to get a pair !
did you use yours in Bi amping/Bi wiring or have you bridged them?
i would like, i don't know if it's possible..?
Bridge mine as Mono for 400watts (with 2 pair of output binding post to Bi wire my MBL speakers..?) to keep the power in 4ohms as final Goal.
Did you have any idea to do that without affect the reliability or it is complicate Job from your point of view?
Could you, if you have, send me some Schematic diagram with technical data sheet & value (ALL you have about this Beloved Graaf!),
cause the original Graaf user manual than i have is really Poor in instructions!
Best & Kind Regards
Friendly Yours
Fred
PS= I dream to test The Destiny VZN350 Joule electra based on 6C33C (as the lamented TENOR 75) and the Atmasphere MA2 but there is quasi none in europe...have you ever heard the Silvaweld OTL 200 base on 6C19 tubes..?
Did you know SI audio ? an italian OTL manufacturer & the Fedelta hifi review owner
http://www.siaudio.it/index.php?option=com_content&view=category&layout=blog&id=76&Itemid=127
here is a good deal on 40 pl504's
http://www.ebay.com/itm/8-x-PL504-Ei-YUGOSLAVIA-BEAM-POWER-TUBE-NOS-NEW-/262000899096?hash=item3d00787418
Hi Friedrichabert !
Check on this Tubes RS site for PL504 tubes , original EI NIS-NOS tubes for which I personally know that quality is very good ,
I known personally that guy( Dragan Nikolic) who sell those tubes and hi is OK guy , but on the other hand I consider that 17E per PL504 tube is way to hot price , but maybe if you by large number of those tubes the net price can go down significantly .
Best Regards !
__
"Art which does not have the appearance of art is true art."
- Old Roman saying -
Hi,
I just purchased a GM200 and agree that it is a superb amplifier.
I've read that one or two of you have a copy. If willing to share it freely, much appreciated. I am currently in Europe on a work assignment and am concerned that technical assistance may be an issue particularly when I return to the US, so would ideally like to have a copy of the service manual.
I will be soon looking into buying a new set of tubes so will be keeping an eye for alternatives on that front. Regards.
Congrats.
Which type is it ? The unbalanced or the balanced one?
I can send you the service manual if you send me your email adress, but not before Monday.
But the manual has little info on how to setup or bias the unit...
Mine was tested and properly tuned by Mariani himself..
Regards
Bernard
Edits: 01/09/15
Thanks Bernard. Appreciated. My GM200 is balanced. It actually came with a manual but a very terse one on how to tune up the amp. I was hoping that there would be a more thorough service manual with diagrams and further specifics on adjusting the amp.. At any rate, happy to take a look at the manual you have (my email is luisavcatao at gmail.com). The previous owner kindly indicated to me the recommended bias and am familiar with both DC offset and bias setting from a previous amp, so guess I can get by with those basics for now.
I am indeed planning to take the amp at some point for some tune up by Mr. Mariani but doubt I will have time for that until the Spring. At the moment what seems more pressing is the decision as to whether to get a tube tester which tests PL504s since I basically have no replacement power tubes, so should one or more blow up I can't operate the amp. Any reference to not-so-expensive tube testers that do a decent job at testing the PL504s are welcome -- I hear great things about the Amplitrex tester but that's beyond my price range at least for now. Also hear good things about the Hickok TV7 and the B&K Dynascan 667 but haven't been able to confirm whether they test the PL504s. Regards, Luis
The best tube tester is the amplifier itself, for the power tubes. If the tube under test is within the limits for voltage across the anode resistor, it's probably "good". That tells you that under the bias conditions prevailing in the amplifier and with the DC voltage provided by the PS, the tube passes an amount of current that is the same or similar to the other tubes. This should be done with all other sockets occupied by "good" tubes, ideally.
Wonder whether someone has used a PL504 brand RFT and/or another called Tungsram in the Graaf GM200? Regards
RCA tubes, especially those made in Japan, followed by Europe, are highly recommended if you can find them.
I also have owned a Graaf Gm 200 and have been very happy with it. It seems as though you have done quite bit of experimenting with the PL504 out put tubes. With your experience have you found any sound difference between different manufactures of the PL504 tube. Also,for sound quality does the set of tubes need to be from the same manufacturer. My amp, which I have had for last ten years, has at the moment a number of different brans of tubes. These included National Electrics CVC Chember and many unbranded. Can biasing the amp overcome any difference with the tubes.I have found a set of 32 PL504 from Valvo made by Mullard. Have you had any expierance with these tubes. Appreciate your advice.
Cheers Ian
Mr.Mariani said that it is very important that the t ubes used are very tight together and of same characteristics- meaning one type. I have a second set of Telefunken and he personaly said that I should not mix them with other made by Philips or others, because they behave different when high demands...
By his words the RCA made in Japan are one of his favorites.... But I could not find any ...
Unfortunately choosing tubes from one manufacturer (e.g. RCA or Mullard) does not always work, unless they are made by the same factory, as there are multiple physical designs of the PL504 tubes that are sold by a single manufacturer. As an example, have a look at some Mullard tubes and you will see that those made in the UK have a getter (silver ring at the top of the tube) that is oriented vertically, while those made in Holland are horizontal.
You want to try and get tubes from the same manufacturer and manufactured in the same way (e.g. all Japanese manufactured RCAs are made the same way, as far as I know).
Does that mean that tubes can be different brands but need to be physically
the same. Also when matching tubes does the whole set of 32 need to be the same value.
They should be the same means either same producer and same type or different labels but same producer and type. Mr. Mariani said that this is of great importance, so the tubes perform at every level the same.
Also the tubes should be matched in octets. You have two octets on the left and two on the right side. How exactly he measures them I don't know. He has them running for a while then measuring again.
My set of Telefunken is waiting for teh day to come at his home.
You can still contact him on this email:
info@graaf.it
They are again in bussines. :)
Hello Mark,
thank you very much for the useful informations you posted. I just bought a used GM200 and I would like to get the service manual.
Is it possible for you to email me the pdf?
Thanks in advance and best regards.
Hello to Mark S Allen and all.
I must ask your comprehention and tolerance with my english. It is not my mother language.
I read with much interest your long explanation on tuning up the Graaf 200 OTL.
I have one too. But the matching procedure is very long and very time consuming. I already did Graaf method but I canīt match well.
I was working with 65mA on Bias, but now have lowered a little bit further to 50m, just for precaution.
I have an Hickock 600 tube tester that tests the 27GB5 /PL504.
Most of the spare valves I have reads between 4000 & 5000 and try organise them like that in the amp, but when I try Graaf matching procedure direct in the amplifier ( those 15,5 & 17,5mV) everything get wrong. Is just like this proceedure contradicts the Hickock.
Most of the 4000 valves read 19-20mV and some 5000 reads 8~10mV. It crazy, and I donīt understand why.
Can you help Mark? It seems you have a lot of experience in this amp. I have other OTL amps that sounds much better. Perhaps I am doing something wrong. My speakers are Quad ELS 57 and YAMAHA NS1000M.
I would recommend that you match tubes by:
1) Doing a basic test with a tube tester to make sure that the tube is in working condition
2)Putting the tube into the amp itself at a set tube position in the amp when the amp has been set up at the correct bias and then measuring the voltage across the anode resistor of the new tube when the amp is on. Write down the voltage across the anode resistor for each tested tube and this can then be used to grade your tubes so that you can select similar tubes that have been tested under operational conditions.
If you put in a new tube in every time you switch on the amp to listen to it, and you listen once a day, you will have tested 30 tubes by the end of the month. I have had tubes measure almost 0 V across the anode resistor (i.e. tube is virtually not operating) and the amp works quite happily with the remaining 15 tubes in the channel spreading the load and showing 1mV extra across their anode resistor. The amp will actually happily work with one of the tubes being blown (red LED showing next to the tube).
While on this subject, I had the opportunity to visit the Graaf factory in Modena about a year ago and see Mr. Mariani fix a GM 200. Not something I would recommend but he happily disconnected the anode clip on a PL504 tube, unplugged the tube, plugged in a new tube and reconnected the anode clip, all WHILE THE AMP WAS ON without any problems (no speakers connected). As I believe there is about 150V on those anodes there is NO WAY I would do the same, but he certainly did, being careful not to touch anything else (comes with 40+ years of playing with tube amps, I suppose :-) ).
Here some photos of the beast.
Edits: 04/07/13 04/07/13 04/07/13
That's nice, glad you are enjoying your amp, mine is still sitting in the box beneath my workbench, where it will probably rest for a long time, I was hoping to get it up and going but too many other projects are in front of it. I have a box of over 80 nos sylvania's for it too.
Maybe someday...
I have heard from Mariani (GRaaf owner) that the 32 pl504 should be matched in octets!!! So maybe 80 tubes is not enough to get it right even for one side....
He has a very strickt policy on that, and I think other that "knew" better just gave a bad reputation on the realibility of this otl-s. And this is not fair.
Edits: 04/10/13
I have a copy of the entire manual on my home computer, I'm away on
business for a few more weeks, but I can send it thru e- mail when I
return.
About the tube matching, from my personal experience I use an amplitrex
tester to test all tubes for plate and mutual conductance, just last year I
tested 50 6lf6's for my futtermans, they use 6 per amp, I ended up with 7
matched sets from the 50 tubes, 8 tubes were either too high or too low to
fit in to the sets.
I installed the worst of the sets into a pair of amps, aligned set bias and
balance and let them work together for 200 hours before I listened to music
or turned them off, that's 8 days. I then listened to them for a few months
about another 200 hours then retested the tubes, they were matched to
within 1 milliamp of each other when retested, much tighter than when I put
them into the amps.
The way the tubes work together in circuit seems to dial them in tighter
just by being in circuit.
Therefore I feel 80 tubes for the Graaf should yield 2 complete tightly
matched sets for the amp. 32+32.
Hello Bryan,
I need a little of your knowledge about Graaf GM200 to try to fix mine!
I find, on power board a little part who became very Hot( not very far of R28 , a name part on board is " MV1" ), I dont find on schematic, this reference neither his value..?
I supose this part is CTP but could you give me value and reference of this MV1 part, and perhaps you could send me better schematic information with technical diagram as you purpose kindly in your message..?
I tried to contact info@graaf.it but nobody answer even by phone...unfortunately.
Could you help me in checking on your GM200's or schematic/technical data sheet?
Many Thanks for your help
Apologize me to disturb you but i don't have any other way as Graaf & Mr Marianni doesn't reply anymore...
Best & Kind regards
Fred
Hi, I would really love to gett this manuals if possible. :)
I had the rare opportunity to see and test the new GRAAF GM400 before the launch in summer or autumn this year!!
All I can say is that it is awesome, the first transistor with a human OTL like sound. As great as all of the Mariani concepts.
Hi, I would really love t get this manuals if possible. :)
That would be great if you could send me the manual.
here my adress info@keber.si
Thanks in advance.
Where did you get the pl504 in large qnt?
I heard from Mariani that they should be matched in 4 octets and that the Japan made were the best(different brands but it was always the same factory).
I first must set it up for proper voltage,but I have no manual only some pictures of the wiring and a code for the wires how it is done.
Another one with the RCA logo...
I also found a nice gm200 checked personaly by mr.Mariani himself. The PL504 tubes used, by his words one of the best and rare are RCA.
What only makes me think is that the unit has only the unbalanced input. I always thought it was balanced only??
I hope it will be able to drive my Magnepan 20.1 speakers, I know that this is not the best match, if it will be to hard on the tubes, I can still use my MC2 mc1250 amp on the bass and the Graaf on mid/ high. Will report how it turns out.
Edits: 04/07/13 04/07/13 04/07/13
Thanks for all the INFO.....I would like to see the schematics if an when you can post..Any info on OTLs you have helps
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