Got my blue glass JJ KT88's

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RJF

RJF

Active member
Been wanting to go with KT88's in the Herbert for a long time now and finally made the move.

I felt a little priced out of the SED 88's so I went with what seemed to be people's second favorite, JJ's. Got them in this morning and am pretty happy with them, the amp definantly seems less compressed on channel 3 than it did with EL34s which is what I was mainly after. THe bottom end almost seems a bit tighter too. I can tell the power section is giving me some serious headroom.

The only thing that confused my tech and I was the bias directions on the back of the amp. We took the fuse out and with the stand by on the bias went sky high and with the stand by off it was dead. We just biased it with the fuse in to around 52ma and called it good. Is this right? :confused:
 
Also, what comes stock for preamp tubes? Mine had 2 Ruby HG's and 4 TAD preamp tubes. Seemed a little odd to me. I believe I put a fresh balanced Ruby HG in the phase inverter. But just to check, which one requires the balanced preamp tube and, which one would it be looking at the chassis from the rear?
 
IIRC, mine came with two TAD JJ ECC83's and four TAD Chinese 12AX7 HG's.

The bias needs to be re-checked. You cannot just set each one to 52 mA because the other two sets under
load will throw them off. I let the tubes warm up for thirty minutes then remove all three fuses, with a speaker load
and throw the standby switch. I then go from test point to test point until all three duets are biased/balanced at my
target or in your case 52. It's a balancing act to get them perfect so you can also use three screwdrivers like I do so
it's much quicker. When you get them all close it's much easier to get them all very close.
Check them again in about a week or 24 Hours of use to see if they drift after burn in.
 
Why were we able to bias it with the fuses in though? With the amp running and playing we are around 52ma on all three pairs. When we tried to bias a pair with the fuse out the bias on that pair just continued to climb until we pulled the lead at 300ma.

You are saying without the fuses pulled I am not seeing the true bias, even when checking while say, someone is playing the amp through a cab?
 
RJF":3g36djjf said:
Why were we able to bias it with the fuses in though? With the amp running and playing we are around 52ma on all three pairs. When we tried to bias a pair with the fuse out the bias on that pair just continued to climb until we pulled the lead at 300ma.

You are saying without the fuses pulled I am not seeing the true bias, even when checking while say, someone is playing the amp through a cab?
Click to expand...

Unless you have six probes on your bias meter, then no.
 
My tech and I are still confused about this removing the fuse and biasing thing. I just rechecked the bias and had dropped to the low 40's on all three. I rebiased, with all the fuses in, to 52ma and did have to re-adjust as I turned each one up to get all three to match. Unless someone can explain the biasing procedure better, I am going to leave it.
 
When you pull the fuse then you are reading (if doing correctly) the plate current with your DMM in series with the plate, which is exact reading.

With the fuse still mounted you are reading in parallel, which (depends on your multimeter) may give "faulty" results.
 
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