Fender DRRI bias range mod

[Randall]

I don't have the amp yet, but I just got a call from a guy who likes modern TungSol 6V6s in his amps, but says they won't bias more than ~ 20mA in both of his DRRI amps. He wants me to get him to 25mA. I told him he might accomplish this by trying a hotter matched set, but he'd rather do the mod. Looking at the schematic, I wonder if the quickest path to this would be to lower the value of the 10K R59 between the adjustment pot and ground? Would tacking in a 10K across it be going in the right direction?

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Enzo]

That would lower it alright.

But really, ten seconds with a resistor and clip wire and then you tell us.

[Randall]

As I look at a Allessandro pulled DRRI board stamped 2010 that I have, there is a 22K in that spot, so I will be interested to see what is in his amp.

[Leo_Gnardo]

I don't have the amp yet, but I just got a call from a guy who likes modern TungSol 6V6s in his amps, but says they won't bias more than ~ 20mA in both of his DRRI amps. He wants me to get him to 25mA.

Twenty is plenty. Crustomer must have read somewhere on the interwebs that more is merrier. He must love to buy new TungSol 6V6's. If he insists, let him. Make sure he won't come whining back to you when his outputs wear out early. I could see going 25 mA on JJ's, they can take it, but really not necessary with them either.

[Enzo]

Revision A - 10k
Revision B - 10k
Revision C - 10k
Revision E - 22k


Looks like the later revision they were going for even cooler.

[Tone Meister]

Well a mA figure is meaningless without plate voltage. The Tung Sol 6V6 is a 12 watt tube, so with typical DR voltages 25 mA would be awfully hot. Even with 420 Vp they'd draw 10.5 watts each. Good luck with that lasting very long.

18-20 mA is typical for most DR and DRRI and again, you need the Vp to calculate an accurate bias setting.

Of course, we all know that but no one spoke up. I feel important now.

[Tone Meister]

Twenty is plenty. Crustomer must have read somewhere on the interwebs that more is merrier. He must love to buy new TungSol 6V6's. If he insists, let him. Make sure he won't come whining back to you when his outputs wear out early. I could see going 25 mA on JJ's, they can take it, but really not necessary with them either.

Around here we treat the JJ 6V6 as a 14-watt tube. https://www.jj-electronic.com/en/6v6s

[Randall]

The first thing I told him is that sounded pretty hot to me, so I am covered here. Actually, he asked me if I could make it go as high as 25 - 30mA.

[Jazz P Bass]

The first thing I told him is that sounded pretty hot to me, so I am covered here. Actually, he asked me if I could make it go as high as 25 - 30mA.

The guy is blowing smoke out his a@@.
I would tell him no, I will not do it.

In the end he may end up respecting you more than if you crippled his amp.

[Chuck H]

Bias the amp in the safe zone and tell him:

"I made an adjustment so that the tubes will perform at the higher current you want when conducting signal, but still give the tubes a break at idle."

Then ask him:

"How does it sound?"

[Leo_Gnardo]

"I made an adjustment so that the tubes will perform at the higher current you want when conducting signal, but still give the tubes a break at idle."

I can see crustomer asking "What exactly did you do?" ("So I can boast about it on the interwebs page where I learned more is better.")

[Chuck H]

I don't have a plan for the 'second question' contingency

[g1]

"the re-issues don't use the secret bias numbers that the originals did, I set it up like a real Blackface".

[Randall]

He has a bias probe, and sets and plays around with it. He says the Russian Tung-Sols sound best to him, so already I think he is full of Internet. He is convinced his amp running at 20mA (don't know what the B+ is) sounds cold to him. I don't think he is aiming towards the 30mA bias point, but just thinks he needs the range. I think.

[Leo_Gnardo]

He is convinced his amp running at 20mA (don't know what the B+ is) sounds cold to him.

Let him try playing his Russian tubes in Russia, he'll find out what cold is right quick.

A customer with a bias probe is a dangerous thing... You could show him where the adjustment pot is, let him set it anywhere he wants with the proviso you are out of the blame loop when things go wally. OR ... stick a resistor in series with Fender's current sense resistor so it looks like 25 mA when it's really 20. There's your secret mod!