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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Music Man Sixty Five
Just had one of these lovely old amps across the bench, and was extremely
surprised to see a plate voltage of 730 on the two 6CA7 / EL34 output pentodes. That is some serious HT, and I'm sure rather higher than I remember seeing typically on any similar amps. The problem was low distorted audio, and was down to an O/C screen (G2) feed resistor. Full normal operation was restored once this had been replaced. G2 volts were around 360 when it was running normally. The bias was about -36v. I actually looked this configuration up, and it just falls within the maximum specs for those tubes, which are quoted at 800v on-load plate voltage max. At this level of plate, G2 and bias, the quoted max output is 100 watts. That's pretty good going from a single pair. Most amps from that era used two pair to get that sort of power. I guess the name "Sixty Five" relates to this amp's claimed output power, but even that is PDG from a single pair, given that most amps of a similar configuration are typically only capable of 30 watts or so. The output tubes that are in it look quite old, but they are delivering the goods, and are pretty reasonably balanced, so I am reluctant to recommend replacement as a standard 'service' option whilst it's in, given the dubious quality and reliability of many tubes these days, even when running at relatively low levels of plate voltage up to say 500v. Anybody know much about these amps ? Not terribly common this side of the pond - or at least I've not had the pleasure of seeing many from this manufacturer over the years ... Arfa |
#2
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Music Man Sixty Five
On Wed, 22 Sep 2010 18:13:49 +0100, Arfa Daily wrote:
Just had one of these lovely old amps across the bench, and was extremely surprised to see a plate voltage of 730 on the two 6CA7 / EL34 output pentodes. That's what they run em at. What did you need to know about them, I used to own a 112 combo 65/35 watter. Probably very similar. -- Live Fast, Die Young and Leave a Pretty Corpse |
#3
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Music Man Sixty Five
"Meat Plow" wrote in message news On Wed, 22 Sep 2010 18:13:49 +0100, Arfa Daily wrote: Just had one of these lovely old amps across the bench, and was extremely surprised to see a plate voltage of 730 on the two 6CA7 / EL34 output pentodes. That's what they run em at. What did you need to know about them, I used to own a 112 combo 65/35 watter. Probably very similar. Nothing particularly. Their website has all the schematics for free download, and I fixed all the problems on this one. I was just interested to see if anyone had any stories about them - or the manufacturer in general - as it's not a brand commonly seen here. Arfa |
#4
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Music Man Sixty Five
On Thu, 23 Sep 2010 01:53:57 +0100, Arfa Daily wrote:
"Meat Plow" wrote in message news On Wed, 22 Sep 2010 18:13:49 +0100, Arfa Daily wrote: Just had one of these lovely old amps across the bench, and was extremely surprised to see a plate voltage of 730 on the two 6CA7 / EL34 output pentodes. That's what they run em at. What did you need to know about them, I used to own a 112 combo 65/35 watter. Probably very similar. Nothing particularly. Their website has all the schematics for free download, and I fixed all the problems on this one. I was just interested to see if anyone had any stories about them - or the manufacturer in general - as it's not a brand commonly seen here. Arfa I didn't care for the sound of my 112 combo. It was tolerable with some external FX. Played a sand colored 130 watt head didn't care for it. Company was founded incognito by Leo Fender who at the time had a 10 year non-compete clause with CBS. The early amps were better, newer being hybrid with tube output. Pedal Steel guitar players did like them however. And they made a decent keyboard amp. -- Live Fast, Die Young and Leave a Pretty Corpse |
#5
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Music Man Sixty Five
"Meat Plow" wrote in message news On Thu, 23 Sep 2010 01:53:57 +0100, Arfa Daily wrote: "Meat Plow" wrote in message news On Wed, 22 Sep 2010 18:13:49 +0100, Arfa Daily wrote: Just had one of these lovely old amps across the bench, and was extremely surprised to see a plate voltage of 730 on the two 6CA7 / EL34 output pentodes. That's what they run em at. What did you need to know about them, I used to own a 112 combo 65/35 watter. Probably very similar. Nothing particularly. Their website has all the schematics for free download, and I fixed all the problems on this one. I was just interested to see if anyone had any stories about them - or the manufacturer in general - as it's not a brand commonly seen here. Arfa I didn't care for the sound of my 112 combo. It was tolerable with some external FX. Played a sand colored 130 watt head didn't care for it. Company was founded incognito by Leo Fender who at the time had a 10 year non-compete clause with CBS. The early amps were better, newer being hybrid with tube output. Pedal Steel guitar players did like them however. And they made a decent keyboard amp. Interesting. This one was a hybrid using some early can-type opamps in the front end. The schematics showed three different output configurations, two of which employed FETs in the driver/phase splitter. The version that I was repairing had a conventional 12AX7 phase splitter. I don't play guitar myself, but keep an instrument in the workshop for test purposes. I have to say that the sound from this amp was 'harsh'. Not distorted or anything, just not the 'smooth' sound that you normally associate with a tube output stage. not 'melodic' sounding is about the best way I can put it. I guess that's why they might be considered good keyboard amps. I've always thought that the 'hard' sound of transistor power amps, suits keyboards well. Arfa |
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