Individual Variable Bias on all Four Tubes

[David Root]

I have started a #124 and want to put a sort-of-set of old used RCA blackplates I have in it that aren't exactly a quad matching-wise.

Anyone done this? Is it overkill? I know I'll have to add a small board with the extra pair of coupling caps and grid resistors.

[talbany]

I've done this on several 100 W and love this one.... I did one for Zac Brown as he liked the older tubes... It's the one in his video Chicken Fried...
Run your bias supply in parallel to each bias pot out of each pot through 220K to coupling cap at that point run the wire to the grids.......Other side of the coupling cap goes to pi plates and split them up in pairs... Simple as that..You have to go back and forth as there is some interaction as well as finding the right value tail resistor for each pot depending on the tube...You may need to use a 1K resistor on the bias board to keep the voltage up or negative voltage I should say.....Most of these I did I built on a separate board using tweaker pots and some I mounted grid resistors on the board as well..That's your call.. Sorry I don't have any pics of that amp as it went out pretty quick.. PM me if you need any help..It's a really cool mod..If you really dial the tubes in to the ma smooth as butter...It takes a while as the tubes settle but to me thats MONEY..!!! Good Luck


Tony

[heisthl]

This might work with RCA Blackplates but under load most tubes conduct differently regardless of how you set the idle current.

[JD0x0]

This might work with RCA Blackplates but under load most tubes conduct differently regardless of how you set the idle current.

+1 also checkout the weber 200 watt amp schematic and layout if you need to see how its done

IMO it's not really worth the trouble but i also think that NOS powertubes arent worth the money. Preamp tubes im a bit more lieniant with although i havent purchased any NOS i found an OLD RCA and its in my ODS sounds better than the tungsol RI is all i can say

[David Root]

I wouldn't pay $800 for a quad of NOS RCA blackplates either, but I got these used ones some years ago for $10 each so I'm not complaining!

[Bob-I]

I'm in agreement with JD0x0. I had an issue with a Super Reverb clone where the bias wouldn't balance. When I put in the dual bias controls the bias was good, but at volume the output was imbalanced. The best balanced output was with the bias voltages the same in spite of the imbalance in idle current.

[David Root]

Interesting difference of opinion. Explains why Dumble did single bias adjust. Perhaps transconductance of each tube is in play here?

I have another amp under work now, my sort of Dumbleized 6SL7 100w 5F6A with two JAN Sylvania 8417s. I have the PI balanced to 1% on both AC and DC, and separate bias adjustment which I have adjusted for the same cathode current. I will try setting them for the same bias voltage and see. Not sure this will do much as currently they are within 0.5 V or so.

OTOH if I do put individual pias pots on the #124 build I can always adjust them for the same bias voltage voltage or the same cathode current.....

[JD0x0]

i think Dumble used single bias adjust because he built GUITAR and BASS amps not HIFI amps. Contrary to what most believe the tubes dont need to be matched or biased perfectly evenly in guitar amps. some players prefer the tone of slightly mismatched tubes with slightly uneven bias.

As long as it isnt too far off that it'll damage the transformers or tubes it's fine. Theres someone over at ceriatone forum with a sovtek and a JJ 6L6 in his OTS because he dropped one of his tubes. It sounds and runs fine.

Also on ODS amps the output section is rarely being overdriven which makes it matter even less and is the reason why most ODS amps are biased on the low-ish side

[talbany]

tubes dont need to be matched or biased perfectly evenly in guitar amps. some players prefer the tone of slightly mismatched tubes with slightly uneven bias.

Have you ever played some that are perfectly (Balanced) bias wise...Not saying it's better or worse just subjective...This lets you do both.. Try them both ways and decide for yourself... I like them as close as possible...


Sure individual bias controls won't make a bad sounding,worn out or gassy
tube sound good or last longer and would no doubt be a good idea to always test them if you can before going through all the trouble...

Tony

[glasman]

I have used split bias adjustments on all of my amps. One thing that you have to make sure is that the 220K grid resistors (bias feedpoint) are at AC ground. If you dont do this then you create a variable high pass filter for each output tube (due to the action of the bias set resistors). Adding a small e-cap at the bias feed to the 220K's to ground will correct the issue. The only downside is that you need to make small adjustments as the caps take some time to charge.

[glasman]

FWIW, I looked at the 200W weber schematic and it has the same issue.

Each of those wipers on the bias adjustments should have a cap to ground to keep the 100K resistors reference to AC ground.

Gary

[David Root]

glasman, how "small" are these caps? talbany, my blackplates are used but test OK on my maxitester. Still plenty of flashing left.

[glasman]

I use 22uf @100v for each feed.

[David Root]

So it passes the entire signal. I should have thought of that. Thank you.

In the TUT books 3 and 5 O'Connor shows a 220K resistor between each bias pot input and the wiper. What is their function? I can only think they are a failsafe in case the wiper drops out.

[Structo]

Hmmm, I didn't know about that AC ground thing, I just installed two bias pots with the same overall resistance as the single bias pot and tail resistor.
Mine are 25K pots and a 15K resistor to ground.
Seems to sound alright.

I'm not sure I follow what Gary is saying.

Where do you put these capacitors and why are they necessary?