A friend has purchased a pair of these monoblocks and has asked if he can fit KT88 in place of the existing 6550 (4 on each monoblock for 110W output)
Can anyone provide a schematic or any knowledge of if it is cathode biased / fixed biased etc.
Any info gratefully received.
Thanks,
Ian
Can anyone provide a schematic or any knowledge of if it is cathode biased / fixed biased etc.
Any info gratefully received.
Thanks,
Ian
I have two of the 110 Watt stereo amps (YM-100) with four 6650s and a 6AN8 per channel. These amps are based on one of the circuits in the Dynaco transformer catalog and are fixed bias. When I first got these I contacted the manufacturer here in California and asked for some info but didn't get very far. First I was told they are NOT fixed bias but I knew different. All other info I have found says their amps are cathode bias. Maybe mine are early ones as there are no serial numbers in the serial number block on the back. Anyway all you need to do is remove the bottom and check the size of the resistor on pins 1 and 8 of the output tubes. Mine are 10 Ohms which definitely means fixed bias, cathode bias would be 500 Ohms give or take. I think KT88s would be fine here. The circuit is very simple and it would be no problem to draw a schematic which I would love to see, hint, hint.
Craig
Craig
Bring out your dead (thread).
These monoblocks arrived at my door yesterday - one is blown. A quick look inside shows one of four 6550 has part melted screen resistor and another tube has completely "fried" screen and cathode resistors.
Anyone have schematics? Just being lazy -I will trace it if no response. The quick look inside showed 6AN8 front end (pentode voltage amp to direct coupled concertina splitter), 4 off Svetlana 6550C Output tubes which were originally cathode biased but has been converted to fixed bias at some point in the past, very neat job so maybe done at the factory.
Cheers,
Ian
These monoblocks arrived at my door yesterday - one is blown. A quick look inside shows one of four 6550 has part melted screen resistor and another tube has completely "fried" screen and cathode resistors.
Anyone have schematics? Just being lazy -I will trace it if no response. The quick look inside showed 6AN8 front end (pentode voltage amp to direct coupled concertina splitter), 4 off Svetlana 6550C Output tubes which were originally cathode biased but has been converted to fixed bias at some point in the past, very neat job so maybe done at the factory.
Cheers,
Ian
Download the "Dynaco Transformer Catalog", my amps pretty much resemble the 120W using the A451 OPT excluding the tube rectumfriers. Do your transformers have any manufacturers names on them? Mine were made by McCarren here in Southern California.
Craig
Craig
Craig,
Done - That helps - Output tranny wires color codes are the same as the A451 and circuit is VERY similar to the 120W schematic. Will complete the trace and post - may not get to it for a while, 40 degrees C (104 degrees F) today and next 5 days are forecast at over 37 degrees C or higher so my workshop out the back of the house may not be a good place to be.
Cheers,
Ian
Done - That helps - Output tranny wires color codes are the same as the A451 and circuit is VERY similar to the 120W schematic. Will complete the trace and post - may not get to it for a while, 40 degrees C (104 degrees F) today and next 5 days are forecast at over 37 degrees C or higher so my workshop out the back of the house may not be a good place to be.
Cheers,
Ian
Checked the transformers. No manufacturer name or any sort of identificatuion or part number on them, however, the power supply choke under the chassis definitely has McCarren printed on it.
Started the schematic trace.
Cheers,
Started the schematic trace.
Cheers,
The Pentode Section of the 6AN8A has me a bit concerned. The tubes fitted are Sylvania.
Should G2 really be 27 to 30 Volts, that seems a bit low. The anode voltage of 77 to 80V would seem correct to bias the direct coupled concertina correctly.
Cheers,
Ian
Should G2 really be 27 to 30 Volts, that seems a bit low. The anode voltage of 77 to 80V would seem correct to bias the direct coupled concertina correctly.
Cheers,
Ian
It's about right...
Is = (500-32) / 1.5M = 0.389mA
Ia/Is ~ 3.4x per datasheet
so Ia = 1.06mA
Ra = (499-77) / 1.06m = 398.65K
Pot = 398.65K - 270K = 128.65K
I think Ia/Is may not be 3.4 anyway, so it's close enough for rock n' roll 😀
Jaz
Is = (500-32) / 1.5M = 0.389mA
Ia/Is ~ 3.4x per datasheet
so Ia = 1.06mA
Ra = (499-77) / 1.06m = 398.65K
Pot = 398.65K - 270K = 128.65K
I think Ia/Is may not be 3.4 anyway, so it's close enough for rock n' roll 😀
Jaz
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Guys,
More info. The unit I traced above has been modified.
Mod #1 - Power Supply beefed up considerably, of all the electrolytic capacitors shown hanging off the rectifier, ONLY C8 to C15 shown on sheet 1 are original. All those "extras" shown on Sheet 3 have been added.
Mod #2 - The single output tube bias control has been replace with individual controls for each tube. Original looked like the Dynaco 120W circuit from here:
http://www.the-planet.org/dynaco/Misc/Transformers.pdf
This mod is poorly done in that the 100K adjustment pots are too high a value and very fiddly to try to adjust. 20K pots with the bottom fixed resistors adjusted accordingly (to say 82K) would be much better.
On ething which "pushed one of my buttons" was that the amp which had blown up had a 2.5 Amp fuse fitted in place of the normal 1 Amp which shows that the last guy to work on them was seriously "dodgy". He then had the cheek to put "Professionally Serviced by ***** " stickers on the rear panels. That is not what I wouild call professional.
Cheers,
Ian
More info. The unit I traced above has been modified.
Mod #1 - Power Supply beefed up considerably, of all the electrolytic capacitors shown hanging off the rectifier, ONLY C8 to C15 shown on sheet 1 are original. All those "extras" shown on Sheet 3 have been added.
Mod #2 - The single output tube bias control has been replace with individual controls for each tube. Original looked like the Dynaco 120W circuit from here:
http://www.the-planet.org/dynaco/Misc/Transformers.pdf
This mod is poorly done in that the 100K adjustment pots are too high a value and very fiddly to try to adjust. 20K pots with the bottom fixed resistors adjusted accordingly (to say 82K) would be much better.
On ething which "pushed one of my buttons" was that the amp which had blown up had a 2.5 Amp fuse fitted in place of the normal 1 Amp which shows that the last guy to work on them was seriously "dodgy". He then had the cheek to put "Professionally Serviced by ***** " stickers on the rear panels. That is not what I wouild call professional.
Cheers,
Ian
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Well they are finally both back together. New set of Output tubes for the blown up unit. Due to the grid leak resistor values (270K) I biased them very conservatively at 60mA idle each tube.
Both 6AN8A had weak triode sections (the concertina phase splitter) so put new RCA 6AN8A in both monoblocks.
Ran them for 2 hours on my system for a final confidence test.
What did I think of them ? Well frankly I far prefer my 10W per Channel 6V6 Baby Huey. Better Imaging, better pace, less low end "covered" veil, better top end extension, well better everything except maximum volume.
Cheers,
Ian
Both 6AN8A had weak triode sections (the concertina phase splitter) so put new RCA 6AN8A in both monoblocks.
Ran them for 2 hours on my system for a final confidence test.
What did I think of them ? Well frankly I far prefer my 10W per Channel 6V6 Baby Huey. Better Imaging, better pace, less low end "covered" veil, better top end extension, well better everything except maximum volume.
Cheers,
Ian
Relocate the ground sos it's not running across the (entire) case and better caps.
It will become heavenly and continue to warm your room on a cold night. pc
It will become heavenly and continue to warm your room on a cold night. pc
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