Single ended EL34 and KT88 interchangable amp

WolfRabbit

WolfRabbit

New Member
Hello! I am working on an El34 amp that should be easily interchangeable with KT88 valves. I bought a single ended 10 watt per channel stereo amp board a while ago from eBay. It was pretty cheap, and I was really curious of the quality of it. Turns out it is actually a really nice pcb with a very clean design, so I plan to go onwards with it. After talking to the seller/mfg, I should just have to change the 2 power resistors to a slightly higher value and wattage to adapt to KT88s. (330 3 watt to 390 5 watt) Would I run into any problems if I use a double pole switch to switch between resistors to be able to easily interchange? I know I would be vastly under driving the KT88s, but I'm curious about the sound differences. Is it really worth adding a resistance switching setup?
 
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You can twist the leads of the transistor and then you can bend it in the direction you need. Hold the lead that you want to bend with some needle nose pliers at the base. then twist the lead with another pair of pliers. That way you wont compromise the integrity of the lead where it joins the transistor body. But i agree with Jeff that it is better to wire your amp point to point. You will appreciate the circuit design and layout much better when doing it point to point. Also you will have more options as to parts that will be limited by the position of the holes on a circuit board.
 
Rather than switch between 330 Ohm and 390 Ohm resistors, use a 330 and 62 Ohms in series (392 Ohms), with the switch shorting the 62 Ohm one to give 330 Ohms. Or if it's easier to connect, a 390 Ohm and a switch to add a 2.2K in parallel (giving 331 Ohms).

Reason is that if there's a switch, sooner or later it WILL get flipped with the power on. If you're switching between 330 and 390, there will be a open position during switching with ZERO current flow. Current going from say 70 mA to ZERO will cause a voltage spike in the output transformer that could arc it over. Either scheme I suggested will result in a much smaller transient, which should be safe (although the POP in the speaker will remind the user to NOT touch that switch while it's on).
 
Tom Bavis said:
Rather than switch between 330 Ohm and 390 Ohm resistors, use a 330 and 62 Ohms in series (392 Ohms), with the switch shorting the 62 Ohm one to give 330 Ohms. Or if it's easier to connect, a 390 Ohm and a switch to add a 2.2K in parallel (giving 331 Ohms).

Reason is that if there's a switch, sooner or later it WILL get flipped with the power on. If you're switching between 330 and 390, there will be a open position during switching with ZERO current flow. Current going from say 70 mA to ZERO will cause a voltage spike in the output transformer that could arc it over. Either scheme I suggested will result in a much smaller transient, which should be safe (although the POP in the speaker will remind the user to NOT touch that switch while it's on).
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Thanks! This helps a ton. I'll definitely be doing this! I just got into the chassis I will be building into today. (I've made a post on this amp before) Upon further inspection, the power transformer does not have the 240v tap I need. Do you know anywhere I may be able to trade for one with the taps I need?
 
Will be watching this thread with interest, I recently obtained a SET/UL amp that uses both these OP tube options...never have used the 88's yet. Good luck with yours :) Al
 
AlTinkster92 said:
Will be watching this thread with interest, I recently obtained a SET/UL amp that uses both these OP tube options...never have used the 88's yet. Good luck with yours :) Al
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Sorry I haven't been on this thread much, I'm on a bit of a standstill. I have everything I need to complete this one, but the power transformer I have (a massive Stancor from a Heath a9) does not have the supply taps I thought it had! I need a 240 supply and the 6.3 filament supply, as well as a pair of output transformers. I even have a nice chassis for it!
 
Thanks Wolf, good luck with your project, my amp is now down at Dave G's for some design work, no NFB and distortion as shown by Dave's Oscope. Al
 
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