• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

KT88SE - with EL34s

Hi all,

Could I just have a confirmation please?

I have the KT88SE build, as per Kegger/Blueglow schematic. Cathode Bias resistor is 500R set up for KT88.

The schematics state a number of other valves are drop in replacements, including the EL34.

Would I need to up the cathode resistor to 560R to ideally suit an EL34? Or, can use them with the 500R.

Having to do a repair which members here (rayma and others) have been very helpful and EL34's (being a half/third of the price of a KT88) are very appealing as funds are a little restricted to be honest.
 
Hi Jeff,

Thanks for your response. We have a thread here but it does contain blueglows scheme art so i'm not sure if there is any infringement, but the schematic is here anyway.

https://www.diyaudio.com/community/threads/kt88-set-internal-wiring-layout.328694/

My plate voltage is 425v,

Cathode voltage 41v with KT88

Also, is it the case that set up for KT88 is not ideal for EL34, but set up for EL34 (e.g. 560R) it would be OK to use a KT88 also?
 
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Also, is it the case that set up for KT88 is not ideal for EL34, but set up for EL34 (e.g. 560R) it would be OK to use a KT88 also?
It might be ok with a 560R at the cathode, but you're definitely running a bit hotter than the original, which had less B+, and was biased quite cool. I'd be tempted to go a bit higher than 560R. 620R would be the next value it looks like.

It's a big jump down to an EL34. Why not try something like a 6L6GC, or KT66? You wouldn't need to make any changes to your amp.

jeff
 
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The schematic voltages stated are from the original Abdellah circuit (with different components values). The final kegger/blueglow schematic has the component values shown as the link, but b+ was 460v and plate was at 450v, as the attachment (I do hope Blueglow is OK with this he did a super YouTube build that was really well presented and helpful)
 

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I can not imagine using Schade negative feedback unless the input tube . . .

Either is a Pentode *

Or is a Triode, with a single un-bypassed self bias resistor. That raises the triode operating plate impedance, rp. **
Using a low impedance LED for most of the bias voltage lowers the operating plate impedance, rp (relative to self bias with a single resistor and no LED).
Schade negative feedback generally works best with high plate impedance, rp.

And yes, doing the above (* or **) will require several resistor values to be changed. Time to re-calculate.

"All generalizations have exceptions." - me

Just my opinions
 
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All generalizations have exceptions." - me

Just my opinions
Well, as much as I would wish, I don't think I will ever be so learned on this subject, too complex for me, way over my head.

Keeping things, like me, simple 🤓 Jeff has given me dilemma.

EL34,
KT66,
6L6GC

I must admit, I've never had the pleasure of an EL34 and would really like to try it. General consensus being EL34 are musical, midrange forward, 'bloomy' whereas KT88 can be solid state(ish), but more balanced, full bandwidth.

Then, of course, the major issue of branding and in all realism, I'd be looking at Shugaung EL34 and KT66 or Reflector 6L6GC
 
It's a big jump down to an EL34. Why not try something like a 6L6GC, or KT66? You wouldn't need to make any changes to your amp.
Hi Jeff, (all)

I've, finally, made my mind up and gone with Genelex Gold Lion KT66's.

Could I please just double check that I won't have to make any adjustments to the Cathode Resistor, which is 500R at present, with a plate voltage of 425v?

Sorry to be a pain just a quick double check, the KT88 i used previously was at 82mA which was running very close to 90%%, quite hot and I'd like to cool things down a little.
 
Hello, how are you operating the tubes, Triode, UL, or Pentode? The added current thru the screen grid affects the overall dissipation of the tube. While KT88 and EL34 have similar high voltage tolerances, the KT88 can handle a lot more current. How did you determine that your power tubes are running at 82mA? When you say Plate Voltage is 425v, is that to ground or cathode? Have you measured the amp yourself? High Voltage Safety is of utmost importance, but you must be able to measure your project amp to determine if you are damaging your tubes. Here's a video from Uncle Doug on measuring the voltage across the cathode resistor to determine amperage and dissipation:


Once I get data from a new amp, I run the data thru the Bias Calculator over at RobRob: https://robrobinette.com/Tube_Bias_Calculator.htm which says your KT88 tubes are running at 85% with a plate-to-cathode voltage of 384v, Vk at 41v, and a 500R Rk.

there's also a simulator here: https://www.vtadiy.com/loadline-calculators/power-stage-calculator/ that's not quite as accurate but fun to play with.

Anyway, it looks like you're ok running tubes like EL34 and KT66 with the 500R bias resistors installed, but you've gotta measure the amp yourself to be sure. Have a pair of 560R ready to swap if needed. Don't forget that your output transformers need a load when the amp is on. Either speakers or a dummy load, I have a pair of 8R 10W I always put across the speaker terminals before I even remove the cover. Be safe, have fun!

will
 
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Hello, how are you operating the tubes, Triode, UL, or Pentode? The added current thru the screen grid affects the overall dissipation of the tube. While KT88 and EL34 have similar high voltage tolerances, the KT88 can handle a lot more current. How did you determine that your power tubes are running at 82mA? When you say Plate Voltage is 425v, is that to ground or cathode? Have you measured the amp yourself? High Voltage Safety is of utmost importance, but you must be able to measure your project amp to determine if you are damaging your tubes. Here's a video from Uncle Doug on measuring the voltage across the cathode resistor to determine amperage and dissipation:


Once I get data from a new amp, I run the data thru the Bias Calculator over at RobRob: https://robrobinette.com/Tube_Bias_Calculator.htm which says your KT88 tubes are running at 85% with a plate-to-cathode voltage of 384v, Vk at 41v, and a 500R Rk.

there's also a simulator here: https://www.vtadiy.com/loadline-calculators/power-stage-calculator/ that's not quite as accurate but fun to play with.

Anyway, it looks like you're ok running tubes like EL34 and KT66 with the 500R bias resistors installed, but you've gotta measure the amp yourself to be sure. Have a pair of 560R ready to swap if needed. Don't forget that your output transformers need a load when the amp is on. Either speakers or a dummy load, I have a pair of 8R 10W I always put across the speaker terminals before I even remove the cover. Be safe, have fun!
What an informative post thank you very much Will.

The amp is in UL mode and I did the measurements a few years ago, correct voltage, as you state, is 384v plate-to-cathode.

I will get 560R resistors as a reserve, and will keep the new 500R I have if I revert back to KT88's in future.

I'll install the KT66 upon its arrival, measure the voltage drop on the cathode, use the robrobinette calculator.

Typically, what is the target dissapation % - 85%?

My dummy load is a pair of old speakers that are 6R, which I believe is still OK for (not too extended) testing purposes.

As a DIY'er, I do take safety seriously thanks for the 'be safe' comment. I wear rubber boots, rubber gloves, goggles, ear muffs (defenders) and use a 5m extension cable. Test time, I plug the amp in, then switch on the mains 5m away!

I've had a few projects smoke and kaboom on me in the past so this system works for me. 🤓
 
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Good deal. You could also swap your 5AR4 rectifier with a 5U4GB, as long as the first cap in your power supply is 32uF or lower. This will lower your b+ by about 60v, possibly creating a better range for the lower-dissipation tubes. That way you don't have to change the circuit at all. Just a thought.

w
 
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Quiescent: The screen voltage on most UL amplifiers is higher than the plate voltage (voltage drop from the UL tap to the plate tap)

Screen dissipation ratings:

6L6GC 5 Watt
All other 6L6 types 2.5 Watt

6CA7 / EL34 8 Watt

KT88 original ratings 6 Watt
KT88 modern ratings 8 Watt

KT66 Design Max 3.5 Watts, Absolute Max 4.5 Watts

Be sure to check when you do a 'Plug and Play' Tube Rolling session.
Otherwise it might become 'Plug and Pray'.

Oh, and filament draw varies from 0.9A to 1.6A depending on which of the above tube types.
Check your power transformer filament current ratings.
Filament windings that were made for KT88 will put more than 6.3V on any of the 6L6 types, including 6L6GC.

Have Fun!
 
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Thanks for that. I had noticed the Vg2 was a bit higher than the plate. I've read its desirable to keep Vg2 below Va in a Pentode circuit, is it not a problem in a U/L circuit? I usually put a 100R 1w resistor on Pin 4 of a U/L circuit, or maybe a but more to keep that Vg2 down...

w
 
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My recent design Balanced Push Pull amplifier has been tested with 6L6GC, KT66, and 5881. I designed it to do that.
I tested it with KT88 too, but I will not run it that way, the filament windings get too hot, not made for 1.6A x 2 + 0.3A input tube.
KT77 and EL34 would work, but again, the filament current would be 1.5A x 2 + 0.3A input tube.
A new power transformer would allow the KT77, KT88, and EL34 to be used.

Do not forget to wire pin 1 to pin 8 for the EL34.
And be sure to check that pin 1 on the KT88 does not connect to the metal shield that is just above the octal header of the tube (a connection there would put the self bias voltage on that shield).
 
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Do not forget to wire pin 1 to pin 8 for the EL34.
And be sure to check that pin 1 on the KT88 does not connect to the metal shield that is just above the octal header of the tube (a connection there would put the self bias voltage on that shield).
I'm definitely going with the KT66 and, presently, there is no connection on pin1 in my amp. So, everything will stay the same, other than, possibly the cathode resistor (500R/560R)


I usually put a 100R 1w resistor on Pin 4 of a U/L circuit, or maybe a but more to keep that Vg2 down...

Indeed,. I do have the 100R resistor on pin 4...
 
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