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Replacing EL34 with KT88 in 50W PP
#1
In looking at the latest schematic for the 50 watt PP it seems like I'd be able to just drop in the KT88s and re-bias accordingly, is this correct? I also read that the "bias feed resistors" might need to be lowered to accommodate a tube other than the EL34...

Thanks in advance!
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#2
Some things to concider with this swap would be these.
The 88s need .100 amps more per tube for the heater, this may or may not be a problem.
If your PTs heater voltage holds at no less than 6.1 volts all should be well with no smoke.

Yes, the 88s will need more negative voltage to get them back into there proper idle current range.

The 88s need more signal drive voltage than the 34s so the amp will go into output stage over drive latter with the 88s.
The dynamics of the two tubes is a fair amount different also.
Once you have enough drive signal hitting the 88s they will go full clip a noticeable amount faster than the 34s.
This also means that if the amps PT supply current output is pushed to its max with the 34s, than the amp will be more spongy when flat out.
Another side fact is that the 88s are over twice the OD of the 34s so you need the tube socket spacing just to pop them in.

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#3
So... Bringing this topic back to life... keeping to the schematic, what R and C values should be swapped with and which Hammonds should be used? This can go to 60w or 70w easy (?)
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#4
(11-28-2019, 06:53 PM)AxelKay Wrote: So... Bringing this topic back to life... keeping to the schematic, what R and C values should be swapped with and which Hammonds should be used? This can go to 60w or 70w easy (?)

I'm  looking at the spec sheet for KT88 and i THINk it says 4.5K Ohms is needed from the OT and  power would be about 66 watts?  If that's the case , hammond's web site will give you the OT you'll need that has those specs.

Good luck

Sam
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#5
(11-30-2019, 02:37 PM)sam.morris1963@gmail.com Wrote:
(11-28-2019, 06:53 PM)AxelKay Wrote: So... Bringing this topic back to life... keeping to the schematic, what R and C values should be swapped with and which Hammonds should be used? This can go to 60w or 70w easy (?)

I'm  looking at the spec sheet for KT88 and i THINk it says 4.5K Ohms is needed from the OT and  power would be about 66 watts?  If that's the case , hammond's web site will give you the OT you'll need that has those specs.

Good luck

Sam
I think the kt88's need about 5.5k ot and 320 0 320 (again, I think ) the C and R values are what make me scratch my head. It's become more of an itch and headache to be honest
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