kt88's in a 2203

Marshall Growl · Post by **Marshall Growl** » Mon Feb 19, 2007 4:51 am

I'm going to throw some kt88's into my 2203, is there any mods that I would need to do besides just biasing?

wdelaney72 · Post by **wdelaney72** » Mon Feb 19, 2007 1:24 pm

Here was an explanation I got for 6550 v. EL34. KT88 tubes would be consistent with 6550. I apologize, as I can't recall who sent this to me, but it was one of the veterans on the forum here...

The bias circuit, for EL34s (B+ around 450V or so depending on wall voltage and bias) it should be 220k in series with the diode in the bias circuit and 56k in line with the bias pot, plus 220k for the grid load/bias return resistors. For 6550s 150k, 47k, 150ks respectively. The R in series with the bias diode gives more negative voltage if you go lower, the R in series with the bias pot gives less as you go lower, and the grid load/bias return Rs won't effect the bias voltage but can have influence on the stabilty of the outputs. The R in series with the bias pot and the bias pot itself make up a resistance from C- (the bias voltage) to ground--the less the resistance is the less negative voltage it gives. The fixed R (47k or 56k) in series with the bias pot makes sure the adjustment range is restricted and you don't short out the C- to ground running the tubes too hot and damage them. The R in series with the diode looks kind of odd maybe brown red (sort of looks orange but maybe it's actually yellow?). If it was 120k, I think could mean that it has a lower B+ PT as found in JMP 50Ws so that was the case 120k would make sense (bit lower B+, bit lower negative bias voltage range). If you want to know what the resistances are, the best thing is to just measure them (amp unplugged, voltage drained, etc.).

Marshall Growl · Post by **Marshall Growl** » Tue Feb 20, 2007 12:48 am

thanks, but just one question. If the bias is in range, do I need to do those resistor swaps?

wdelaney72 · Post by **wdelaney72** » Tue Feb 20, 2007 1:42 pm

If you are going from EL34 to KT88, I can't see that even being remotely possible.

Marshall Growl · Post by **Marshall Growl** » Tue Feb 20, 2007 8:08 pm

Ok, thanks for the advice man

Mr Crumb · Post by **Mr Crumb** » Fri Feb 23, 2007 11:22 pm

wdelaney72 wrote:If you are going from EL34 to KT88, I can't see that even being remotely possible.

Help me out here. Why is this not possible? The 6550 is the US version of the KT-88. Conversion form el34 to 6550 and visa versa is a simple bias mod. So why not the KT88?

Marshall Growl · Post by **Marshall Growl** » Sat Feb 24, 2007 12:07 am

He's saying it's not possible if I don't change the resistor if not several.

wdelaney72 · Post by **wdelaney72** » Tue Feb 27, 2007 8:22 pm

By design EL34 and 6550 differ in:
1) Plate Dissipation
2) Screen Grid Voltage
3) Contro Grid Voltage
4) Cathode Current

6550 are a different, more robust tube. They run differently. When Marshall opted to switch USA models to 6550, they adjusted the bias circuit accordingly. Not because they wanted to as it was an added expense, but because it's required for the tube to be biased properly. KT88 are the same as 6550, so a jump from EL34 to KT88 would need the same changes.

Marshall Growl · Post by **Marshall Growl** » Wed Feb 28, 2007 2:47 am

All I did was change from a 56k to around 70k. But, on my other thread, Wdelaney says you should do more than this, so it has me a little worried. They'r in range now to run them at about 28 watts at idle. So, I've gotte some mixed oppinions on this.

wdelaney72 · Post by **wdelaney72** » Wed Feb 28, 2007 9:56 am

Marshall Growl wrote:All I did was change from a 56k to around 70k. But, on my other thread, Wdelaney says you should do more than this, so it has me a little worried. They'r in range now to run them at about 28 watts at idle. So, I've gotte some mixed oppinions on this.

I'll have to defer to Larry, Dan, Steve or some of the other true electroncs experts. I believe it was Dan (Billy Batz) who I think sent me the info I listed earlier on the changes needed.

novosibir · Post by **novosibir** » Wed Feb 28, 2007 3:05 pm

You still have to swap the 220K bias feed R's with 150K's

Larry

Marshall Growl · Post by **Marshall Growl** » Wed Feb 28, 2007 10:51 pm

Thanks everybody

MajorWatt · Post by **MajorWatt** » Fri Mar 02, 2007 10:44 am

On my '68 Metro board, I just changed the bias voltage input resistor to a 4.7K and I was able to get between -70 and -80 vdc.

novosibir · Post by **novosibir** » Sat Mar 03, 2007 8:19 am

Just to make it clear once again, based on Marshall's technical informations for use of 6550A/KT88 tubes in the 1959 and 2203 you have to change alltotal 4 resistors:

- both the bias feed resistors from 220K to 150K
- in the bias supply the 27K from the white cable to the diode to 15K
- the 56K in front of the trim pot to 47K

That's all and you will have the needed bias adjustment range and the safety margin for the output tubes, to avoid blocking distortion.

Larry

Marshall Growl · Post by **Marshall Growl** » Sat Mar 03, 2007 1:04 pm

Hey, thanks

Looks like I'll be doing that tonight.