Fisher 400 Bias P/S Resistors

[MouseMaster]

I have an early version 400 that I have just started to rebuild. I have the Metalbone kit, and have started with the bias power supply. The serial number of my unit is 11015B. I found unusual values for two resistors in the bias power supply. According to the prints, R102 should be 5.6K. In my 400, R102 is 830 ohms. R94 should be 10K. My 400's R94 is 37K. It does not appear to have been altered in the past, and it did not redplate when I last powered it up briefly last week. Anyone here ever run into anything like that with a 400? I took close up pics. I will try to post them up....

Here they are. The first ones are after the new components are installed. Next is of the original components. Also some of the unit itself showing the general condition. Most of the original Telefunkens are still there, although one or two test weak. The outputs are RCAs with a little getter wear.

In the first photo, R102 is the vertically oriented upper-most resistor. R94 is the horizontally oriented resistor beneath it.

 

[audiodon]

Yes,
Schematic errors or factory fine-tuning that varied from the schematic was common then. I've seen that in at least 2-3 Fisher 400s. I wouldn't worry about it now. When you're done, you may have to tweak the values to bias it up at the 7868s to your liking.

 

[MouseMaster]

Thanks Don, appreciate the reply.... I'll check voltages next time I apply power.
:yes:

 

[MouseMaster]

Got this puppy running tonight.

I used the Metalbone kit with upgraded output coupling caps (Sonicaps). I did not replace any bias resistors. I think the outputs are tired. I have 420V on the B+, and -19 on the bias, -19.5 after running an hour or so. All four output tubes are dead on with one another for bias voltage. Idle current on the outputs ranges from 10ma to 18ma. Three are ranging 16.8 ma to 18ma, and one is at 10ma. That one has more brown in the getter area.

The power transformer was a little warm after an hour, as was the 4 section can. The other two cans were cool, and I have already tested the 100uF 300V insulated can as OK. That 4 section can is definitely gonna go as soon as I can get one, or a suitable sub. Probably dump the others too, just to be sure nothing bad happens.

Again, this is a very early unit, s/n 11015B.

I only have it on small 4" 3-way Radio Shack speakers right now, but it still sounded pretty good in spite of it.

What do you guys say? Time for a fresh quad on the outputs? At least two X7's have to go as well -- weak and/or unbalanced halves.

 

[dsndblm]

The quad of Fisher labeled 7868s that came with my 400 were tired as yours are. They worked but sounded bad with any kind of volume. I hunted around and found 6 Conn labeled Sylvania 7868s for a decent price. I was able to make a decent matched quad out of the 6 Sylvania's. I've heard good things about the Electroharmonix 7868s but have also heard that the pins are bigger in diameter and stretch the tube socket contacts.

Don't be too hasty to get rid of those marginal Tele 12AX7s. I've used the weaker tubes in V9, and V10 with no noticeable difference. V11 and V12 need good, strong tubes. If you aren't going to spin any vinyl put the marginal tubes in V7, V8. Or, you could send them my way and I'll take care of them for you.

 

[audiodon]

7868s are kind of like 7591s in one way.
For both, the tube can look great but not hold a bias or run at high current.
For both, the tube can look awful but hold a fixed bias rock solid and work fine.
Don't discard them because they look tired. If it holds bias solid (the same after an hour as after a few minutes) you don't have to rush to replace them.

 

[autoteck1]

Electroharmonix 7868s

The Electro-harmonix 7868s look close to original and they sound great like the originals, But they definatly do have larger pins and will strech the sockets so that they will not hold stock tubes. They fit so tight infact it is allmost impossible to get them in the sockets without cracking the glass ! I think the next time i use a set I will pre strech the sockets with a suitable diameter drill bit(upside down) as I did crack one, it fractured after about 20 min. of playing. Thank God they replaced it free of charge. I have these in a resocketed x-101-b ,they sound exactly like the original tubes ,been in there for about a month ,so far GREAT !!

 

[Destructor]

Anyone ever remove an EH7868? Seems like one could crack upon removal they are so tight.

 

[audiodon]

I've removed them.
Flipped it upside down, removed the bottom chassis cover.
Mild rocking plus pressure on the bottom with a non-conducting plastic piece I got in a shoebox did it fine.