7189 vs EL84 +458v plate

kkillebrew · 27th January 2009, 09:06 PM

Hello,

I've got a Scott LK48 that uses 7189s in the output and measures +458v on the plates. I'm currently using a quad of 7189s branded as Mitsubishi, don't know what they really are.
I'd like to get another set of output tubes and am looking for suggestions. Wondering how some 6bq5/EL84s might work... +458v is kinda high. Finding NOS 7189s is difficult and they are very pricey.

The amp is running a pair of stacked ESL57's.

m6tt · 27th January 2009, 09:38 PM

So how many volts are on the cathode? (assuming cathode bias, could be wrong)...recent amp I did had 415v at the anodes, with 45v at the cathodes (and it's still in AB1, I swear!)...that means the tube is only "seeing" 370v, not 415.

Also, 6p14p-ev. I believe they will work well.

Ty_Bower · 27th January 2009, 11:24 PM

Quote:

Originally posted by m6tt
Also, 6p14p-ev. I believe they will work well.

At 400+ volts, the 6p14p-ev is your best, most economical bet. I've run them at 410 volts cathode bias and they are perfectly happy. Just watch the plate dissipation and you'll probably be OK.

planet10 · 28th January 2009, 02:31 AM

fixed bias...

dave

kkillebrew · 28th January 2009, 09:36 PM

It is a fixed bias amp and the cathodes are grounded. Not exactly the same schematic that Planet10 shows as mine uses 7199's for the driver splitter, but the output bias is the same.

So how does the neg bias voltage effect the overall plate voltage load? Lets say it's fixed biased at -40v (haven't measured) and the plate to ground voltage on the 7199s is 450v (which it is). Is the 7199 plate seeing a 450v potential or 410v? Or in between?

kenpeter · 28th January 2009, 10:33 PM

I've been a bit busy yet to fry some 6p15p in my Fisher.

Need to verify first the "unused" internally connected
pins don't short out anything the real 7189s wouldn't.
And arrange safety precautions that might protect
me and the rest of the amp in case of G2 failure....

XRays of several similar pentodes of this family have
been posted to another thread. I don't see any huge
difference of internal geomety to explain why some
are characterized for higher screen voltage.

But it could be some quality of materials or vacuum
that isn't able to be seen even by XRay photograph?
I still suspect there may be no difference at all.

m6tt · 28th January 2009, 10:53 PM

fixed bias does not affect plate volts. So it's still 450v. The maximum voltage rating is generally anode to cathode (aside from heater to cathode voltage ratings), so if the cathode's at 100v and the anode at 500v, the tube is only seeing 400v. A cathode resistor, a la cathode bias, develops it's bias voltage as a result of the voltage drop across it, and *is* subtracted from B+. This makes the cathode positive leaving ground (where the grid is at) as an apparent negative voltage (with respect to cathode). So you can drop volts by developing some or all of the bias by cathode resistor, but in fixed the tube will "see" 0v at the cathode and 450v at the anode.

kenpeter · 28th January 2009, 11:27 PM

As I recall, both channels of mine are dropped across one string
of four heaters... But its been more than a week since I glanced
at the x100a schematic, I could be imagining something wrong...

27th January 2009, 09:06 PM	#1
kkillebrew diyAudio Member Join Date: Jan 2009	7189 vs EL84 +458v plate Hello, I've got a Scott LK48 that uses 7189s in the output and measures +458v on the plates. I'm currently using a quad of 7189s branded as Mitsubishi, don't know what they really are. I'd like to get another set of output tubes and am looking for suggestions. Wondering how some 6bq5/EL84s might work... +458v is kinda high. Finding NOS 7189s is difficult and they are very pricey. The amp is running a pair of stacked ESL57's.

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27th January 2009, 09:38 PM	#2
m6tt diyAudio Member Join Date: Aug 2007	So how many volts are on the cathode? (assuming cathode bias, could be wrong)...recent amp I did had 415v at the anodes, with 45v at the cathodes (and it's still in AB1, I swear!)...that means the tube is only "seeing" 370v, not 415. Also, 6p14p-ev. I believe they will work well.

28th January 2009, 09:36 PM	#5
kkillebrew diyAudio Member Join Date: Jan 2009	It is a fixed bias amp and the cathodes are grounded. Not exactly the same schematic that Planet10 shows as mine uses 7199's for the driver splitter, but the output bias is the same. So how does the neg bias voltage effect the overall plate voltage load? Lets say it's fixed biased at -40v (haven't measured) and the plate to ground voltage on the 7199s is 450v (which it is). Is the 7199 plate seeing a 450v potential or 410v? Or in between?

28th January 2009, 10:33 PM	#6
kenpeter diyAudio Member Join Date: Nov 2007 Location: Dallas	I've been a bit busy yet to fry some 6p15p in my Fisher. Need to verify first the "unused" internally connected pins don't short out anything the real 7189s wouldn't. And arrange safety precautions that might protect me and the rest of the amp in case of G2 failure.... XRays of several similar pentodes of this family have been posted to another thread. I don't see any huge difference of internal geomety to explain why some are characterized for higher screen voltage. But it could be some quality of materials or vacuum that isn't able to be seen even by XRay photograph? I still suspect there may be no difference at all.

28th January 2009, 10:53 PM	#7
m6tt diyAudio Member Join Date: Aug 2007	fixed bias does not affect plate volts. So it's still 450v. The maximum voltage rating is generally anode to cathode (aside from heater to cathode voltage ratings), so if the cathode's at 100v and the anode at 500v, the tube is only seeing 400v. A cathode resistor, a la cathode bias, develops it's bias voltage as a result of the voltage drop across it, and is subtracted from B+. This makes the cathode positive leaving ground (where the grid is at) as an apparent negative voltage (with respect to cathode). So you can drop volts by developing some or all of the bias by cathode resistor, but in fixed the tube will "see" 0v at the cathode and 450v at the anode.

28th January 2009, 11:27 PM	#8
kenpeter diyAudio Member Join Date: Nov 2007 Location: Dallas	As I recall, both channels of mine are dropped across one string of four heaters... But its been more than a week since I glanced at the x100a schematic, I could be imagining something wrong...

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