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Old 16th June 2008, 06:44 PM   #1
bbuz is offline bbuz  United States
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Default 6gw8 /ECL86 El Cheapo modifications

Hello,

I have all the more expensive fixings for an EL Cheapo( chassis, AC transformer as well as very nice pair of OP Transformers....all part of an amplifier currently using 4 very nice Telefunken ECL86 tubes.

Just wondering, if anyone ( Eli Duttman?) would be kind enough to suggest the needed tweaks to use the ECL86 tubes in the circuit instead of 12AQ5's .... ( and 12at7's if that is possible or reccommended...( I have some nice telefunken 12at7's too if there is benefit to using the 12at7+ECL86).

Thanks!

Bill
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Old 17th June 2008, 01:11 AM   #2
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Bill,

The 6GW8 triode looks mighty similar to the 12AX7 triode. It's high RP and low gm, which is exactly what you don't want in a splitter/driver. The best use I can come up with for those triodes is to connect the grids to the cathodes and use the resulting diodes in the vacuum side of the hybrid B- bridge rectifier. Parallel pairs of "synthesized" diodes appear to have adequate current handling capability.

Perhaps you can arrange a swap of the TFK ECC81s in your possession for 1950s vintage black plate RCA 6201s. All TFK made tubes may yield an overly dark voice.
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Old 17th June 2008, 02:44 AM   #3
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The 12AT7 and its distortion pattern are a key part of the sonic signature of El-Cheapo. I really recommend that if you want to personalize the circuit that you choose a different approach. The 12AT7 is pretty much the heart of the audio circuit in that little amp.
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Old 17th June 2008, 03:21 AM   #4
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I've also had the case where the 12AT7's distortion spectra actually reduces total system distortion. A more linear driver made total distortion worse. Depends on the system though, so it isn't always true.
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Old 17th June 2008, 03:44 AM   #5
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Bill,
The 6GW8 / ECL86 are great tubes. If you want to stick with them then have a look at what Yvesm did with them here:
http://www.dissident-audio.com/PP_ECL86/Page.html

and my minor variation:
http://www.diyaudio.com/forums/attac...amp=1113448831
I called this amp the "Baby" and for the last 3 or 4 years it has been driving a pair a Polk R5s in a friends house.

The whole "Baby Huey" thing grew out of Yves design and my subsequent "mucking about" with variations on that design.

My recommendation is that if you have Output Trannies with Ultralinear taps then use my variant of Yves design. If the OTs don't have UL taps then use Yves original design.

I don't want to get into a male primary sexual characteristic waving exersize here BUT I do believe that for 6GW8 these designs will out perform the "El-Cheapo".
A caveat on that: You don't want to run more than +300V to 320V DC for the 6GW8 so check to see if the Power Trannies you got are suitable. Maybe a choke input power supply will suit - model using PSUD2 from Duncan Amps pages.

6GW8 Data Sheet:
http://www.mif.pg.gda.pl/homepages/f...10/e/ECL86.pdf

The 6GW8 amp I built to the circuit above was absolutely gorgeous.

Cheers,
Ian
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Old 17th June 2008, 02:29 PM   #6
bbuz is offline bbuz  United States
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Smile 6GW8/ECL Solution

WOW!!!

Thanks everyone for such quick and helpful responses- this is my first experience with DIYAUDIO.COM and I have to say I am impressed and encouraged.

Ok, well, as too often it seems,the answer to my dilema is "no" "yes" and "both" in that order......

( NO) Eli, as you pointed out the 6GW8 will not workin the El Cheapo, got the feeling it would be a force fit and not ideal... and still need the 12AT7.......thanks for the explanation

(YES) Gingertube, as you were kind to bring to my attention, the BAby Huey IS right for my 6gw8 tubes.....and should excell there so I think I will just modify my current 6gw8 chassis and iron to a Baby Huey.....

(BOTH) Boris and Jim chiming in about the 12AT7 magic has me interested- as it turns out with some more searching I have some nice RCA 12at7's as well as about 50 RCA 12AQ5's .... so I think I will do the EL Cheapo as prescribed- and be contacting Jim to finalize an order shortly.

One last question for all, I will need the following to do the EL Cheapo

Chassis
OP Transformers( I am looking at the Edcor PP X series- are they acceptable?)
AC Transformer (S)

Any input would be appreciated for sources and models that would give good results.

Again, many thank to all for sharing your tube wisdom and experience.

Sincerely,

Bill Buz
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Old 17th June 2008, 02:40 PM   #7
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Default Re: 6GW8/ECL Solution

Quote:
Originally posted by bbuz
One last question for all, I will need the following to do the EL Cheapo

Chassis
OP Transformers( I am looking at the Edcor PP X series- are they acceptable?)
AC Transformer (S)
Antique Electronic Supply will have a chassis and hammond power transformers. Edcor's XPP or CXPP will also do quite well. Edcor also has power transformers, and much more than what is listed on their webpage.

Here is a list of what they have (that we know about) for power transformers. Phyllis will always help you to find an existing part instead of paying the $20 NRE fee for a custom transformer.
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Old 18th June 2008, 12:23 AM   #8
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Bill,

Ian is correct about "Baby Huey" making more sense in an amp based on the 6GW8. I merely indicated how to get "milage" out of 6GW8s employing "El Cheapo" style circuitry.

For an EC based on 6V6 family tubes, the Edcor O/P trafo I suggest is the CXPP25-8-7.6K. That trafo gives you the option to use both triode and ultra-linear (UL) modes.

You can buy a complete set of power "iron" from Allied Electronics. Allied stock # 967-2343 is the B+ trafo. Stock # 967-8019 supplies both "12" V. heater power and B+ boost. Stock # 967-1004 is a suitable B+ filter choke. Stock # 227-0034 is the 6K27VF B- and 12AT7 filament trafo.

Jim McShane can supply all the small parts needed at a fair price. Speaking of Mr. McShane, if you lack the wherewithall to properly pair your 12AQ5s up, you could do FAR worse than paying Jim for that service.

A chassis is easy. Visit a nearby WalMart and buy a large/deep plain Aluminum baking pan.

I suggest you use the TFK ECC81s in combination with the RCA 12AQ5s. Acquiring Mullard made 'T7 family tubes would be excellent too. Using tubes from a single manufacturer can give you voicing problems. RCA is famous for "chime", which is great, but too much chime is not so good. Mullard is famous for a midrange accent, but all Mullard can give you a sound that's too "round". RCA and Mullard in combination is "boffo". TFK and RCA rates to be good too.
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Old 23rd June 2008, 04:44 PM   #9
bbuz is offline bbuz  United States
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Default 6gw8 /ECL86 El Cheapo modifications

Thanks again for the information and explanations.

One question on the transformer recommendation. My impression was that the XPP15-8-8K would be about right for a PP 6gw8 at 15 watts- and be capable of UL since it has the screen taps ? Did you recommend the CXPP25-8-7.6K did you recommend that due to the 7k6 primary ( better match ?), increased wattage capabilites for improved headroom? or a better match to power output from the 2 6gw8 or 12aq5 in PP?

.........................

Can't wait to hear the EL cheapo's & Baby Huey in their glory and will be ordering parts this week( and looking for a parts list. sources recommendations in the achive search)


In the mean time, I was repairing a 6gw8 based amplifier for my brother( replaced old electrolytic caps, bad output transformer( shorted secondary) and bad 6gw8 tube.

It is an old Heathkit AA-32 amplifier( three 6eu7's going into four 6gw8's for stereo PP output) and it is nice, but I would be very interested in recommendations for tweaking the AA-32 circuit-...."HUEYING " or EL CHEAPO'ing" it?

link to schematic:

http://www.nijnkonijn.nl/machmat/sch...p/heatha32.gif

..... adding the CCS or making UL seemed obvious mods... but wondering about voltage changes to the 6gw8's or 6eu7's to improve headroom or sonic performance/ gain staging.

Thanks!

Bill
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Old 23rd June 2008, 04:49 PM   #10
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Default Re: 6gw8 /ECL86 El Cheapo modifications

Quote:
Originally posted by bbuz
In the mean time, I was repairing a 6gw8 based amplifier for my brother( replaced old electrolytic caps, bad output transformer( shorted secondary) and bad 6gw8 tube.

Shorted secondaries are extremely rare. Did you mean primary? If you measure a short on a secondary with a meter, that is probably a normal reading of very low ohms. Most often output transformers fail by open primary. If the primary measures OK, then the tx is probably good.
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