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"favorite" outputs for Simple P/P?

George's last answer helped fit the final pieces of the puzzle on my Simple P/P EL84 project, but made me realize that I'll probably need to invest in new outputs.

All my previous EL84 builds have been triode mode, for which SV83 family devices were fine - actually these can sound rather more than just fine - but due to screen rating are limited in ultimate power in pentode mode - with full B+ voltage on the screens, I doubt they'd last very long.

So, the question is of current production EL84 / 6BQ5 types, any particular standouts or warnings?
 
In my own SP-P, I first installed Electro-Harmonix EL84's. Immediately, I noticed a very slight redness of the plates (B+ around 300v). This phenomenon was also reported by Russ, here on the Forum. I had two quads of the E-H's to try, and all tubes acted the same way. However, the redness never went beyond very slight, even with the amp working hard for several hours. The sound was fine. Very detailed and clean. Will the EH's have a short life? I may find out as I'm still running with them most of the time.

Next I tried (on Russ' suggestion) some JJ EL84's. As Russ predicted, the plates did not glow on these. The sound was also very good. I couldn't really pick them over the EH's. According to guitar players, the JJ EL84's have a rep for toughness (unlike their KT88's which are garbage).

Finally, I bought some 6P14P-EV from a Russian guy I've dealt with before.
These tubes were NOS military issue by the Reflektor plant. They also ran for hours with the amp at high volume, and no distress from the plates. Again, after about 50 hrs break-in, I couldn't detect any substantial sonic differences.

Currently, I'm waiting for a quad of Gold Lion EL84's; another Russian tube that is supposed to have excellent sonics. It better offer something special as they are around $30 each!

I'm starting to think that the Tubelab SP-P design is not as fussy about tubes as some amps are.
I'd suggest you install some of the Russian 6P14P-EV. They can be found fairly cheaply, and appear to sound good and are ruggedly constructed.
 
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All my previous EL84 builds have been triode mode, for which SV83 family devices were fine - actually these can sound rather more than just fine - but due to screen rating are limited in ultimate power in pentode mode - with full B+ voltage on the screens, I doubt they'd last very long.

Why not regulate the screens at 250-300v? That way, if your B+ is a little high (Hammond PT), the tubes will last longer.

jeff
 
Why not regulate the screens at 250-300v? That way, if your B+ is a little high (Hammond PT), the tubes will last longer.

jeff


well, the point of this particular build was to keep things simple - as far as I digressed from bone stock was to upgrade the cathode bypass and coupling caps, and add choke after first filter cap

regulated screen supply and other zoomy tricks will probably be reserved for the next rebuild of the current 'lil Scottie amp or the Jolida/Dynamutt also currently in parts on the workbench

neon: thanks for the tip on the 6P14P-EV, it occurs to me that I might already be using a set of those that I got from Dave in an existing amp.

BTW, just how "left" coast is your location? - so far this seems to be an all maple leaf thread
 
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well, the point of this particular build was to keep things simple - as far as I digressed from bone stock was to upgrade the cathode bypass and coupling caps, and add choke after first filter cap

Sorry, I should have said "unregulated". Old school amps used a simple RC section after the B+ tap to drag down the voltage for the screens.

jeff
 
Would a choke added lower the B+ maybe enough for those EH etc? It shouldn't cost much either and may help PS.

Be carefull with those Gold Lions as I'm sure I read somewhere they won't take high B+ too. I think Jim McShane as a warning on his site as he sells them.

I was using JJ EL84's in an old Heathkit AA-151 which is supposed to run B+ high and they worked great too. It needs recapping so I went back to an Eico HF-12 for now which I think runs the tubes at less B+.

Maybe that mentioned Edcor spec'd 280-0-280 or less PS trans. would solve the problem. A little loss of W/CH though!

Would the JJ's and Russian EV tubes last longer at the lower B+ too?

I bet the Simple PP amp would run 7591 types as the drive requirements are supposed to be similar to EL84? Lots of B+ would be great then for up to 30W/ CH! Maybe 35W if pushed.

Sockets would have to be external as they are Octal, but all the Dynaco ST-70 boards work that way.

Randy
 
Shuguang has a new series of high end 9 pin tubes to complement their Treasure series of Octal tubes. They are called Psvane series and they have an EL84 tube in it. I think they are $67-70 a pair and would be probably compared with the Gold Lions. It is unknown if they can take higher B+.

Randy
 
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Hey Chris,

George's last answer helped fit the final pieces of the puzzle on my Simple P/P EL84 project, but made me realize that I'll probably need to invest in new outputs.

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
 
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.

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.
 
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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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.
 
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
 
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? :eek: 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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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.
 
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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