SSE pentode/triode/UL

[neomat]

Hi guys!!

I may misunderstand something but I'll ask. George website mentionned that SSE can be run on triode, UltraLinear and Pentode. But UL/triode has only 2 positions. So my question, certainly a silly one : how switching to pentode ?
By using a On Off On switch?

Thanks to enlighten me.

 

[rknize]

Triode: screen is strapped to the plate with a resistor
UL: screen is attached to the UL taps on the OPT
Pentode: screen is attached to a fixed supply (would be B+ in this case)

You would need a DP3T switch to do it with just one switch. Not many people bother with pentode operation. If their OPT has UL taps, they use those. If not, then pentode is the closest option.

 

[neomat]

Thanks!
SSE pcb seems not to allow pentode mode. As far as I understand, B+ should be connected to R18/R28 lead going to tube, is that right?

Have a nice day.

 

[rknize]

I think you can leave R18/28 where it is and just connect the board terminal to B+. Be sure to observe the specs of the tube. Make sure that your raw B+ does not exceed the tube's maximum rated screen voltage. Ideally, the screen should have it's own supply. This is because it really should not operate at a higher voltage than the plate. Because of the loss across the OPT primary, your plate voltage is lower than your B+. However if the tube you are using is designed for UL operation, it should be fine.

 

[cjkpkg]

This is an interesting idea...I have been wanting to try pentode operation on my SSE with the KT90's.

I assume then I could hook it up like so...
Unhook the UL taps and tape/shrink them off and run a jumper per the attached.

Obviously a switch could be employed, even a 3 way switch to do Triode, UL, and Pentode if the results warranted.

 

[Tubelab_com]

It works as shown in the above post. The usual audio tubes 6L6GC, KT88, 6550, EL34 and 6V6GT (with a suitable B+ voltage) will work fine. A 6L6GA, or 6L6GB will not work with the usual 400+ volts either.

Keep in mind that the output impedance of a pentode is quite high, so the damping factor will be very low, maybe even less than 1. This can make for some really boomy bass on many speakers. Some feedback is needed to clean up pentode mode, often more than the CFB circuit will provide.

It's not too hard to modify the board to allow for conventional GNFB to the 12AT7 cathode. Schade can be tried, but it didn't sound too great on my speakers. Try a 220K 2 watt resistor from the output tube plate to the 12AT7 plate.

One builder told me that he got a great sounding SSE be eliminating the CCS, using a 12AX7 for the input tube, and applying Schade feedback. This would obviously require some resistor value tweaking, but no details were provided and I haven't tried it.

 

[cjkpkg]

Interesting...thanks George!

Instead of using just a jumper would you recommend using a 560ohm resistor (or so)to knock the raw B+ down below the plate?

I tell you I have really enjoyed the SSE since the last round of updates:
CXSE's
KT90s
200mA Edcor PT
Mullard CV4024 input tube
Jantzen Silver Z coupling caps

I have had it running as UL pretty much the whole time - I tried triode mode with the KT88's that were in there but ended up going back to UL...I have not tried triode with the KT90s.

I am running the 90's at around 95mA if memory serves...

 

[Skorpio]

It works as shown in the above post. The usual audio tubes 6L6GC, KT88, 6550, EL34 and 6V6GT (with a suitable B+ voltage) will work fine. A 6L6GA, or 6L6GB will not work with the usual 400+ volts either.

Keep in mind that the output impedance of a pentode is quite high, so the damping factor will be very low, maybe even less than 1. This can make for some really boomy bass on many speakers. Some feedback is needed to clean up pentode mode, often more than the CFB circuit will provide.

It's not too hard to modify the board to allow for conventional GNFB to the 12AT7 cathode. Schade can be tried, but it didn't sound too great on my speakers. Try a 220K 2 watt resistor from the output tube plate to the 12AT7 plate.

One builder told me that he got a great sounding SSE be eliminating the CCS, using a 12AX7 for the input tube, and applying Schade feedback. This would obviously require some resistor value tweaking, but no details were provided and I haven't tried it.

Looking at the RH84 and RH807 amps, the driver tube is also ECC81 and the feedback resistor is 100k...

How much feedback will 100k and 220k resistor result in? And how much will the gain from EL34 change when going from triode to pentode?

Idealy, you get the same gain and output impedance, but doubled power output

http://www.tubeaudio.8m.com/807/807.html