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Marantz 8b bias power supply

W

whitelabrat

New Member
I'm working on replacing the selenium rectifier in my Marantz 8b's bias power supply. I've followed a schematic that the previous owner had given me. The schematic mentions that the supply output should be -45V. Of course this may vary as each 8b is unique. Mine is rock solid on -43V. When I bring the voltage on my variac up to 117V the bias meter on one pair of tubes is pegged. I can't get it down. When I lower the mains voltage things get more under control.

So should I replace the 43V zener with a lower value? Scrap the zener and replace the 499ohm resistor with a 1.6k ohm? The original resistor value was 1k.

I've attached a schematic.
 

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I'd be inclined to put 2-3 normal diodes in series with the 43v zener, to create another 2 volts of drop... making it 45v total. That should make it easier to deal with...

BUT- if you're ONLY having bias problems with ONE channel, you've got something wrong WITH THAT CHANNEL. Bad coupling cap, bad tube, bad tube socket, bad connection to the bias supply... SOMETHING in that channel is not right. If it was the bias supply, the same problem would exist on BOTH channels...

Regards,
Gordon.
 
Yeah. I though that was odd too. I've used this 8b for a year with no issues until now. The tubes are definitely OK. The only think I can think of is that the bias supply was putting out only -41V or less from the start. It's hard to find Zeners around 40V so I'm thinking of yanking it out and putting in a 1.6k or 2k ohm resistor in place of the 499 ohm. Don't ask why I used a 499 ohm. It's close enough to 500 ohm.

The other channel's bias does seem a bit high too. The other side just seems more sensitive. I still get audio from the couple of seconds I had things going.
 
Regulating bias supply is not a good idea. If it is unregulated, idle current will drop as line voltage rises (since control grid has more gain than the screen grid). Bias supply should only be regulated if screen supply is regulated. (main supply with ultralinear)
 
I have just replaced the selenium in my recently acquired 8B with a 1000 PIV diode and it biased perfectly. No regulation needed.
 
Tom Bavis said:
Regulating bias supply is not a good idea. If it is unregulated, idle current will drop as line voltage rises (since control grid has more gain than the screen grid). Bias supply should only be regulated if screen supply is regulated. (main supply with ultralinear)
Click to expand...

This aspect I had not thought of. On consideration, I have to agree completely with Tom on this.

Restoring the unit to a resistor-based "voltage divider" instead of the zener, would be a good idea, given this logic (which seems to be pretty compelling)...

Regards,
Gordon.
 
GordonW said:
This aspect I had not thought of. On consideration, I have to agree completely with Tom on this.

Restoring the unit to a resistor-based "voltage divider" instead of the zener, would be a good idea, given this logic (which seems to be pretty compelling)...

Regards,
Gordon.
Click to expand...

yeah..me too. I even went so far as to look for the most sensitive g2, in the big low-triode mu TV sweeps...g1 is more sensitive by a smaller margin there, but it is still more sensitive. For an EL34 it is not even close...:)
cheers,
Douglas
 
GordonW said:
This aspect I had not thought of. On consideration, I have to agree completely with Tom on this.

Restoring the unit to a resistor-based "voltage divider" instead of the zener, would be a good idea, given this logic (which seems to be pretty compelling)...
Regards,
Gordon.
Click to expand...

IME it's totally compelling. I experimented with regulated fixed bias supplies on Dyna ST35, ST70 and Mk3. In every case the plate current varied much more widely with fluctuations in line voltage when bias was regulated.

FWIW, I found plate current to vary in a minor way (not inversely) with variation in line voltage when bias was unregulated. Plate current variation with line voltage was unnervingly large when bias was regulated.
 
I've yanked the zener and replaced the resistor with a 5W 1500 ohm I had about. Overkill for the job, but it was all I had in my parts bin. Works like a champ! The bias voltage came up to roughly -45V and all four EL34's biased without a problem. I'm good to go.

I'll post a photo when I get a chance.
 
Since I've got everyone's attention here, I'm curious to know what EL34's ya'll use? I'm using SED, and I'm not sure if I care for them. Even my cheap chinese 6L6's sound better (on a different amp).
 
My unit came with the original Mullard XF2 dual getter halos, all testing very strong.

However, I'll stash them away for special occasions.

Just ordered a quad of SED EL34's for daily use. I've tried them in the past with a Dynaco Stereo 70 with excellent results.
 
Here's the bias:

Marantzbias.jpg
 
Resurrecting this thread...

As I wrote in an earlier post, I had been using my Marantz 8B with a simple 1:1 swap from Selenium rectifier to silicon diode with no problems at all.

However, when I tried a set of KT77s recently, I found that I didn't have enough adjustment range on the bias pots. In other words, I had to set each bias potentiometer clockwise all the way, with no additional room for adjustment.

So it appears the bias supply negative voltage is much higher than spec (-45V). Even though I haven't measured it yet.

I unearthed this thread and fellow member whitelabrat appears to have found the sweet spot using a 1.5K ohm resistor (R35 in the schematic) in place of the stock 1K ohm, when replacing his rectifier. This to compensate the lower voltage drop of the silicon diode.

I guess I'll have to perform this modification in order to properly bias the KT77s.

I'll post pics soon.
 
I went to the local surplus electronics store and found a NOS Allen Bradley carbon composition resistor with 10% tolerance, just like the original.

Settled for 1.6K ohms to replace the stock 1K ohm R35.

Note that the selenium rectifier remains bolted on the chassis to preserve the original look. It is disconnected from the circuit, but one leg is used to attach the resistor, one section of the dual bias cap and the silicon diode.

The silicon rectifier has been cleverly concealed using black shrink tubing so that, at first glance, the unit would look totally original.

With the 1.6K ohm resistor, I am now able to dial in the bias for the KT77s with room for further adjustment.

I wholeheartedly recommend using this value when restoring a Marantz 8B.

I hope this thread is useful for future reference.
 
Last edited:
You guys with Über-valuable old Marantz units need to learn to source better parts.

Xicon caps in an 8b? :shudder: Might as well head for Radio Shack...hell, they might have been able to fix ya up.
 
EchoWars said:
You guys with Über-valuable old Marantz units need to learn to source better parts.

Xicon caps in an 8b? :shudder: Might as well head for Radio Shack...hell, they might have been able to fix ya up.
Click to expand...

Yeah, I think I'd at least be using Illinois caps... they're very little more cost than Xicon.

If it were mine, it'd be Panasonic FC and similar stuff. Or Nichicon... they're not THAT much more expensive!

Though, I will say- I've used many Xicon caps in various stuff, to save money... and as long as you are conservative with their use (don't go over 75% of rated voltage, for one example), they've worked fine... it's when you PUSH them, that things get dicey. And with the Xicons- DEFINITELY pony up for the 105C temp rated caps... the 85C caps are too much of a gamble, IMHO...

Regards,
Gordon.
 
EchoWars said:
You guys with Über-valuable old Marantz units need to learn to source better parts.

Xicon caps in an 8b? :shudder: Might as well head for Radio Shack...hell, they might have been able to fix ya up.
Click to expand...

You are right. I left the original dual cap on mine (it tested good), but I did pick up a pair of NOS Spragues that look authentic, just in case.
 
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