Avalon vt-737sp

[deveng]

Kags: So if you replace only one tube (which seems logical) do you replace with a balanced single?

 

[kags]

I get EH or JJ 6922 tubes from The Tube Store. Prob 30 bucks each.

 

[gyraf]

..go with EH, that's probably the safest bet..

 

[pucho812]

Repaired the burnt traces, replaced BD140 BD139 pair and replaced all electrolytics. The board does work now but I found at least one bad 6922 tube. According to Avalon and their replacement set at Sweetwater, they have a set of 4 matched EH6922's. I can't see why these tubes require matching unless they are matching the two sections in each tube? Is that really necessary? Any suggestions on replacements, EH, JJ etc? The originals are Sovtek

matching tubes was always for peak performance. matching tubes was the first in many stages of testing tubes. Even new ones from the factory could have problems. I would say we would reject some 10% of each tube batch when we would order tubes by the thousands. Luckily they were replaced free of charge and I assume still sold to some other poor sod.

Matching tubes is also great for marketing. As far as tubes in my time there, we were using EH. the reason being that sovtek discontinued the 6922 and the EH was a close match in terms of sonics. I still prefer the Sovtek's and have more than enough to last my lifetime. Even if I give away a ton. I have also experimented with tube rolling using a 6cg7 in place of the 6922 in the V1 position, that is the tube for the input and the closet to the chassis side wall.
As for having 4 new ones because one went bad? Up to you but it doesn't hurt.

As for the center detents on the input and output pots, there are trim pots for that, which in a perfect world would get adjusted after a new tube or set of tubes get installed. What the center detents are for is as follows. any input signal is equal from input to output whth no eq, filters, or compression going on. So for example if I send it a +4 dBu line level signal have the pots at center detent and do not engage any of the eq, hpf or comp, I will measure +4dBu at the output.

 

[deveng]

matching tubes was always for peak performance. matching tubes was the first in many stages of testing tubes. Even new ones from the factory could have problems. I would say we would reject some 10% of each tube batch when we would order tubes by the thousands. Luckily they were replaced free of charge and I assume still sold to some other poor sod.

Matching tubes is also great for marketing. As far as tubes in my time there, we were using EH. the reason being that sovtek discontinued the 6922 and the EH was a close match in terms of sonics. I still prefer the Sovtek's and have more than enough to last my lifetime. Even if I give away a ton. I have also experimented with tube rolling using a 6cg7 in place of the 6922 in the V1 position, that is the tube for the input and the closet to the chassis side wall.
As for having 4 new ones because one went bad? Up to you but it doesn't hurt.

As for the center detents on the input and output pots, there are trim pots for that, which in a perfect world would get adjusted after a new tube or set of tubes get installed. What the center detents are for is as follows. any input signal is equal from input to output whth no eq, filters, or compression going on. So for example if I send it a +4 dBu line level signal have the pots at center detent and do not engage any of the eq, hpf or comp, I will measure +4dBu at the output.

Thanks to all for the info and help. I've got this unit up and working. The I/O board was really burned but I was able to rebuild the connections with wire. I measured all the tube pin voltages with old tubes in, no tubes and nothing appeared odd. Replaced the tubes and all is well.

 

[deveng]

Thanks to all for the info and help. I've got this unit up and working. The I/O board was really burned but I was able to rebuild the connections with wire. I measured all the tube pin voltages with old tubes in, no tubes and nothing appeared odd. Replaced the tubes and all is well.

Was thinking before I send back to customer, does anyone have all tubes in, no input signal, measured grid voltages for all 4 tubes for this unit? Edit: better question would be what are the expected plate currents?

 

[pucho812]

Was thinking before I send back to customer, does anyone have all tubes in, no input signal, measured grid voltages for all 4 tubes for this unit? Edit: better question would be what are the expected plate currents?

Sadly I don’t have that data and no longer have access to schematics. But let me ask, is it doing something odd or unusual?

 

[deveng]

Sadly I don’t have that data and no longer have access to schematics. But let me ask, is it doing something odd or unusual?

Not operating unusual but I just wanted to confirm the operating points of the tube set. Without a schematic (and the tube pcb hiding some of the components on its board and the board below) its not simple to determine. The grids for each section are all negative with respect to the cathode but I'm not sure where these tubes should running. All measurements are with tubes in place with respect to ground, no input signal. If anyone is working on a unit that has the tubes working properly and has time to compare, that would be very helpful. These measurements were taken directly at the solder side pins on the bottom of the tube sockets. Anything look out of place with these voltages?

Tube	Plate #2	Grid #2	Cath #2	Heater	Heater	Plate #1	Grid #1	Cath #1	Int Shield
	Pin 1	Pin 2	Pin 3	Pin 4	Pin 5	Pin 6	Pin 7	Pin 8	Pin 9
V1	98	0	2.3	0	6.3	188	98	100	0
V2	92.2	0	2.4	0	6.3	188	92	94	0
V3	103	-0.78	21.3	0	6.3	188	103.2	104.5	0
V4	186	13.25	15.3	0	6.3	187	38.5	47.8	0