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Old 10th November 2010, 01:25 PM   #11
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The JJ's seem to work the best, sound good, and don't cost a bunch. I haven't tried the 6P14P-EV, but several users report good results.

The 6P14P without the EV will exhibit a slight glow. I don't know how long they will last that way. The 6P14P tube is commonly sold on Ebay as 6BQ5's, EL34's, 7189's and even 7189A's. Thye are not replacements for any of these.

Quote:
Why not regulate the screens at 250-300v?.....I tried to lower the screen voltage on the EH tubes, due to the issues that neon mentioned. I did this with an external power supply. Indeed it brought plate dissipation down, but at the expense of additional harmonic distortion.
I tried this too. In any attempt to lower the screen voltage the power output went down and the distortion went up. I was using two regulated bench supplies for both plate and screen voltage. It seems that for a single supply voltage the sweet spot for sound quality and distortion is 340 to 355 volts. Adding a resistor and cap to drop the screen voltage a bit starts to increase the distortion because the screen current varies a lot with EL84 type tubes.

The simplest way to lower the dissipation is to increase the value of the cathode resistor. I had to use 360 ohms with the 6P14P's that I got on Ebay. This reduced the power output to 12 WPC.

I have found a second sweet spot with the plate voltage up in the 400+ volt range and the screen in the 320 volt range. This REQUIRES a screen regulator. It also requires tubes that can stand 400+ volts without blowing up. JJ's work fine. I can also get a bit more power with some type of cathode voltage clamp like a bunch of LED's (like the RLD amp), big zener diodes, or an adjustable solid state circuit. I can easilly get 25 WPC in this manner with very low distortion, but the P-P is no longer Simple! An amp built in this manner has been in the works for a while, but my construction time has been limited lately.
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Old 10th November 2010, 09:13 PM   #12
chrisb is offline chrisb  Canada
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Originally Posted by vinylkid58 View Post
If it helps you decide, you may borrow my NIB Edcor 25W 8K OPT's. They're waiting patiently for me to finish the top plate for my amp. They are wired with 40% screen taps etc.

EDCOR Electronics Corporation. CXPP25-MS-8K

jeff

Thanks Jeff, I meant output tubes - this amp is using a spare pair of Scott LK48 OP transformers that you've already heard in the "el cheapo" amp - George's answer referred to deriving screen supply voltage, which with "appropriate" tubes can be run at full B+. It turns out that the current working amp is already running Russian 6P14PEV, and Dave has more .


I've already installed little jumpers on the terminal blocks, and now simply need to get around to digging out the rectifier and driver tubes from my stash. Unless I've made some bonerhead wiring mistake, I should be ready to let the music out of this thing by the weekend.


well, maybe I'll bench test it first
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Last edited by chrisb; 10th November 2010 at 09:15 PM.
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Old 10th November 2010, 09:57 PM   #13
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well, maybe I'll bench test it first
NOTE:

Don't connect this thing (or any newly created amp) up to your nice high efficiency speakers. If using GNFB there is a 50-50 chance that the amp will oscillate at full power. It sounds rude on crummy speakers, but may fry something more sensitive. If this happens swap the two primary plate leads.
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Old 10th November 2010, 10:22 PM   #14
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Originally Posted by chrisb View Post
I've already installed little jumpers on the terminal blocks, and now simply need to get around to digging out the rectifier and driver tubes from my stash. Unless I've made some bonerhead wiring mistake, I should be ready to let the music out of this thing by the weekend.
Feel free to bring the amp over for a listening session, one you've got everything ironed out and running smoothly.

I guess I'd better get moving and finish my amp.

jeff
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Old 11th November 2010, 06:55 PM   #15
chrisb is offline chrisb  Canada
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Originally Posted by tubelab.com View Post
NOTE:

Don't connect this thing (or any newly created amp) up to your nice high efficiency speakers. If using GNFB there is a 50-50 chance that the amp will oscillate at full power. It sounds rude on crummy speakers, but may fry something more sensitive. If this happens swap the two primary plate leads.



Sorry if that freaked you George - it was meant sarcastically - I'll likely get a real technician to test it first - all I have is a mutli-meter and dummy resistor loads. I've not yet smoked a driver any of my tubed amps - shorted out mini-Aleph is another story though

Would it be hubris to state that I have no crummy speakers on hand with which to first test the amp? Actually the only speakers in the house with an XO of any type would be a powered computer set - they'd certainly not qualify as musical, but can' be driven directly.


But actually that raises another dumb question regarding feedback - my OPT have 4/8/16 taps, and with our various speaker designs under test, we may well use them all at different occasions. While I've generally preferred my previous P/P triode connect amps sans FB ( a 10yr old Jolida JA302 was U/L only for the first 6 months, at which time the FB was also disabled) , this would be the first pentode amp I've built. It will generally be used to drive moderately efficient FR drivers such as Fostex, Mark Audio, Fountek, etc. What would be the recommendation for feedback?
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Last edited by chrisb; 11th November 2010 at 07:01 PM.
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Old 12th November 2010, 01:06 AM   #16
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Anyone try these yet Genalex - Gold Lion N709 / EL84
I am currently using JJs and very happy with them
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Old 12th November 2010, 01:43 AM   #17
rknize is offline rknize  United States
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Quote:
Originally Posted by moparman View Post
Anyone try these yet Genalex - Gold Lion N709 / EL84
I am currently using JJs and very happy with them
I have not, however I would not expect them to be different in the dissipation category from EH or the other New Sensor brands.
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Old 12th November 2010, 08:05 AM   #18
Ian444 is offline Ian444  Australia
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Chrisb, some possibly useful info here, or maybe not, I've been running a Simple PP in pentode mode and driving CHR-70 in Lotus cabinets. I use no NGFB, but changed the four screen resistors from 100R to 820R to clean up the distortion. The B+ on this amp is around 320V and the OPT's are Hashimoto HWC-30-8. Results may be different for other B+'s or OPT's. That said, most people prefer a little NGFB so just building the amp as it is should give good results, its NFB cct is fairly light, but seemed to kill the sound for me with these particular speakers.
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Old 12th November 2010, 05:27 PM   #19
chrisb is offline chrisb  Canada
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Originally Posted by Ian444 View Post
Chrisb, some possibly useful info here, or maybe not, I've been running a Simple PP in pentode mode and driving CHR-70 in Lotus cabinets. I use no NGFB, but changed the four screen resistors from 100R to 820R to clean up the distortion. The B+ on this amp is around 320V and the OPT's are Hashimoto HWC-30-8. Results may be different for other B+'s or OPT's. That said, most people prefer a little NGFB so just building the amp as it is should give good results, its NFB cct is fairly light, but seemed to kill the sound for me with these particular speakers.

Ian, thanks for the info. In my case the OPT are vintage Scott (LK48) and the speakers are dual CSS EL70 ( "essentially" paper cone version of CHR) in the "Castle Micro-tower". Another EL84 P/P with same OPT (we came across 2 "organ donors") is running in class A triode (no feedback) on the same speakers - sounds great, but a little low on ultimate power, and I think the MA will shine even more those extra pentode watts.

Of course, I'll need to get off my a$$ and plug some tubes into this guy and verify operating voltages before connecting to speakers.


FWIW, I've also incorporated the Hazen mod* on this piece, as did the builder of my EL34SET, and I quite like the results. The former has been running 3-4hr weekdays and 20hrs weekends for at least a year now with no smoke and no detectable deterioration.

*"rediscovered" / announced by Steve Deckert a year or so ago as "Christmas Comes Early" . IINM, he's now including it on all his shipped amps, and still covering with warranty. Of course, it only works if suppressor grid is not internally strapped to cathode, so not all EL34 or 84 types need apply (no harm, the cap will simply be shorted by the internal connection) .

BTW, could you post link or PM me a copy of the schematic that you drew for the SimplePP? Dave e-mailed me a copy a few weeks ago, but I've misplaced my printed copy and the PDF got lost in e-mail black hole.

cheers
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Last edited by chrisb; 12th November 2010 at 05:32 PM.
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Old 13th November 2010, 01:39 AM   #20
Ian444 is offline Ian444  Australia
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Chris, 2nd post here for schematic:
SP-P final connections?
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