Being an intensely happy new owner of a D79C, I came across the following:
The B+ is about 600V and according to ARC, one should bias the 6550/KT88 output stage to 65mA.
I didn't measure inside the amp, as the access to the PCB under the tubes is not exactly easy.
However, if I enter this data in a simple tube bias calculator, the outcome is that the output tubes are stressed to almost 100%.
I do not believe this, and wonder where the mistake is.
-Could it be that there is a rather large V on the Cathode, for instance? For the schematics, I can't see this.
I did, however, measure over the G1 resistor (goes to the meters on the front, so easy to access) and it does indeed bias up to 65mA.
All your comments are more than welcome. I am sure I made a stupid misstep somewhere.
Cheers,
J.
The B+ is about 600V and according to ARC, one should bias the 6550/KT88 output stage to 65mA.
I didn't measure inside the amp, as the access to the PCB under the tubes is not exactly easy.
However, if I enter this data in a simple tube bias calculator, the outcome is that the output tubes are stressed to almost 100%.
I do not believe this, and wonder where the mistake is.
-Could it be that there is a rather large V on the Cathode, for instance? For the schematics, I can't see this.
I did, however, measure over the G1 resistor (goes to the meters on the front, so easy to access) and it does indeed bias up to 65mA.
All your comments are more than welcome. I am sure I made a stupid misstep somewhere.
Cheers,
J.
Actually ARC does run the output tubes pretty hard so you (and that calculator) are not off the mark. Important to use high quality replacements if not purchased from ARC.
I used to service ARC gear until the early 1990s and had a lot of problems with tube rollers in particular. They don't typically listen and repeatedly blew up the amps (M100s mostly) using tubes that could not survive under the existing operating conditions.
I used to service ARC gear until the early 1990s and had a lot of problems with tube rollers in particular. They don't typically listen and repeatedly blew up the amps (M100s mostly) using tubes that could not survive under the existing operating conditions.
There were 4 versions of this amp and they differ significantly in detail.
The output stage consists of two matched pairs of tubes per channel, the ones used in a specific bank must match very closely since a single pot is used to adjust the idle current of both in a bank. (At least in the original version) I believe the 60 - 65mA quoted is for the pair, not individually.. They should be operating well within their dissipation limits.
Note that few modern 6550 operate well at 600V so I would try to get something like the KT88 referenced or the current KT90 also from New Sensor.
It is critical that the pairs in a bank be very closely matched - buying matched quartets is a good idea. (See my comments in the D90 thread)
The output stage consists of two matched pairs of tubes per channel, the ones used in a specific bank must match very closely since a single pot is used to adjust the idle current of both in a bank. (At least in the original version) I believe the 60 - 65mA quoted is for the pair, not individually.. They should be operating well within their dissipation limits.
Note that few modern 6550 operate well at 600V so I would try to get something like the KT88 referenced or the current KT90 also from New Sensor.
It is critical that the pairs in a bank be very closely matched - buying matched quartets is a good idea. (See my comments in the D90 thread)
Ah, I was looking at the schematic and was mislead.. LOL I haven't seen one of these amps in decades.
Which version do you have?
In that case the manual is clearly incorrect as the power dissipation at 60mA is 36W and standard GE 6550 were rated 35W, and the A version 40W.
These amps use cathode feedback derived from the secondary so are sitting quite close to ground.
So depending on tubes you'd be running between 90 - 100% of rating.
Which version do you have?
In that case the manual is clearly incorrect as the power dissipation at 60mA is 36W and standard GE 6550 were rated 35W, and the A version 40W.
These amps use cathode feedback derived from the secondary so are sitting quite close to ground.
So depending on tubes you'd be running between 90 - 100% of rating.
I have the C version (only 25 or 30 made ;-)) and it is weird to see the bias setup like this, for several reasons. The manual clearly states that the amp should be biased at 65mA, and that is exactly the current draw you achieve when dialing in to the spot on the power meters where you should bias it.
Can you imagine how puzzled I am?
By the way: My other amp, a D90 revised with 6L6GC (B+ 410V) should be biased at 60mA. That's kind of the same story. I still hope I am overlooking something. Can't imagine that WZJ designed his amps like this.
Can you imagine how puzzled I am?
By the way: My other amp, a D90 revised with 6L6GC (B+ 410V) should be biased at 60mA. That's kind of the same story. I still hope I am overlooking something. Can't imagine that WZJ designed his amps like this.
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Unfortunately he probably did, the tubes available at the time he designed that amp were pretty rugged, he had a reputation for running them hard which given the better quality of tubes at the time didn't present the issues it does today. The KT88 today would generally be a safer choice as they can dissipate a bit more power.
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I stopped working on them around 1992 and I think the reference series stuff came a few years after that. (I guess mid 1990s) They were terrible to work on and I got the basket cases because invariably I could fix them.
I did work on a very sick D250 MKII probably towards the end of the 1990s, but that was the very last one. (Boy was that heavy!!)
The pre-amps are more reliable and the only ones I ever worked on were the SP-8, SP-10, and SP-11.
I did work on a very sick D250 MKII probably towards the end of the 1990s, but that was the very last one. (Boy was that heavy!!)
The pre-amps are more reliable and the only ones I ever worked on were the SP-8, SP-10, and SP-11.
I found ARC products to be well engineered too. I didn't do warranty on them, but tended to get the basket cases after someone else was in there. One thing you have to watch is the screen grid voltage which is regulated. Everything I saw tended to have a few years on them and had resistor issues as well.
As far as using good tubes, absolutely! But being in the service trade as Kevin and I are, you wouldn't risk your reputation for a couple dollars on some tubes. Besides, they were marked up by a constant percentage so it didn't really matter. Having them in stock was painful though as they did cost a lot. I still maintain a reasonable stock, but not as many as before.
-Chris
As far as using good tubes, absolutely! But being in the service trade as Kevin and I are, you wouldn't risk your reputation for a couple dollars on some tubes. Besides, they were marked up by a constant percentage so it didn't really matter. Having them in stock was painful though as they did cost a lot. I still maintain a reasonable stock, but not as many as before.
-Chris
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