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smbrown 28th January 2008 12:10 AM

Help on Pentode Octal Phono Stage (POPS)
 
I'm restoring an old Heathkit A-7 and I think the phono stage needs help. The amp as a whole has lots of promise. The line input sounds great, even with the load of existing paper caps. I've replaced the electolytics, and a few resistors. I've linked a schematic below.
I think the phono stage sounds good for the most part, but is bass light. I've replaced the cathode bypass with 100uf, and the input resistor w/100k. I've also replaced the screen cap w/0.47uf. I'm wondering if anyone can help with a review of the RIAA values, and/or give me some help on figuring them myself. The other issue is that the gain of this stage is a bit low when used with my Shure M91ED cartridge.
By the way, the amp is amazing in that even with all AC heaters it is still dead quiet, even w/volume wide open. Oh, and here's a couple measurements I've made.. the plate of the 12SJ7 is at 33v and the screen is about 54, which seems odd. The cathode current is about 1.3 ma. I have not replaced the 12SJ7 and I don't have a way to test is, but I do have a new one on the way.

http://www.audiophool.cjb.net/Schem_A/Heath_A-7C.gif

Eli Duttman 28th January 2008 01:33 AM

2 Attachment(s)
Steve,

As you are undoubtedly aware, the A7 came in many "flavors". I'm attaching the A7E schematic, which uses a 12SL7 in a RCA circuit for the phono section. Changing over to that circuitry rates to solve many of your problems.

kevinkr 28th January 2008 02:36 AM

Eli raises an important point about the phono input in your amplifier. IIRC Even the A7-E phono input version possibly predates the current RIAA equalization standard, but the earliest versions with phono pre-amplifiers certainly did. (I've seen more than a few that did not have the phono stage.) What this means at the minimum is that the LF turnover frequency is probably in the wrong place for RIAA. The A7-E design should provide a lot more gain and can be tweaked to get the proper response if not spot on.

I had a pair of these at one point, and the output transformers are quite nice, but the power transformers seem to have shorted high voltage secondaries in about half the units I have encountered. (None recently.)

They're very tubey sounding, but in a pleasant sort of way.

Eli Duttman 28th January 2008 03:52 AM

2 Attachment(s)
Kevin,

I don't think the A7E's phono EQ is problematic. I've attached the "classic" RCA circuit. Notice the CLOSE similarity.

smbrown 28th January 2008 05:39 AM

Thanks for the feedback. I was concerned that the eq was off compared to modern RIAA standards - so that's likely why it doesn't sound right. I guess the best solution is to swap it to a 12SL7, but that is going to require a whole new set of parts. The other issue I've found with this is that the output tubes seem to be run far too hot. The max plate voltage is about 250 for the 12A6, but these are running about 380 and 27ma. I'm sure stuff like that, the lack of a line fuse, and leaky PS caps all contribute to the failure rate of the power transfomers. So far I'm not finding that it sounds all that "tuby" however, sounds pretty clean and open to me, despite the use of feedback and the dated signal caps. I wonder if the different sound from what Kevin has heard is because this one has the outputs run in pentode, vs. UL for the E models?

kevinkr 28th January 2008 01:40 PM

In terms of sound it might just be my frame of reference, they aren't the last word in transparency, but compared to many comparably priced units they are very good...

Mine were the A version and they were also operated in UL. The voltages as you observed are way over the top, and I suspect this was deliberate in order to get a reasonable amount of power out of the 12A6 - Even taking into account today's line voltage increases they were operated very hot. 12A6 were common war surplus at the time these amplifiers were designed, and given the relatively low price of these amplifiers I don't think tube life was a major design consideration..

I think Eli's suggestion for replacing the phono stage is just the right way to go, EQ apparently isn't a concern as the RCA reference design it is based on is RIAA, and in addition to providing the right RIAA, and additional gain, it will probably be a lot quieter (snr wise) than the pentode based design.

smbrown 28th January 2008 03:47 PM

Thanks Kevin, a 12SL7 it shall be! Maybe an octal based Cornet, would work well in there. That takes 1/2 of a 6SN7 for the output of the phono section, but that would not be a problem if I eliminated the tone control function - I'd have a spare socket. As for the output, I'm wondering if a 12V6 is close enough to sub and if so, maybe wire them in triode and get rid of the feedback. Heck, this will be a TOTALLY different beast, but could be a sweet little amp, especially if I can find it a sister for a reasonable cost.

Eli Duttman 28th January 2008 11:12 PM

Steve,

:D :D I'm grinning because much of what you mentioned overlaps with a project I'm involved in. I'm working with a fellow to rebuild an A7 carcass into a HT center channel amp and a stereo phono preamp.

Your thinking about triode wired 'V6s is fine. However, use Saratov, Russia, made TungSol "reissue" 6V6s, with their heaters in series, as the "finals". The "reissue" is better than much of the remaining NOS. ;) Ask Poindexter. Connect your 8 Ohm speaker to the 16 Ohm tap. Voilla!

You can improve B+ filtration and, simultaneously, ease the pressure on the power trafo by employing a 5AR4 rectifier working into a 15 muF. 1st filter cap. Follow the I/P cap. with a gyrator (synthetic inductance) and finish the B+ filter off with a large reservoir capacitance. The extra Volts a 5AR4 yields compensate for the drop in the gyrator.

There are 5 Octal signal tube sockets in an A7 chassis. I suggest you use 2X 12SL7s, a 12SN7, and 2X 'V6s as the tube complement. One 'SL7 forms the RIAA gain and EQ circuitry. The 2nd 'SL7 is set up as a SRPP power amp voltage gain block. One 'SN7 section forms the phono section buffer, while the 2nd section is employed as a "concertina" phase splitter.

Eliminating the tone controls frees space on the front panel, which is used to advantage. A control opening is used for the mounting hardware needed by a toroidal power trafo that feeds a DC heater supply for the 2X 'SL7s. The other control opening is used for a DPDT phono/line selector toggle switch. Remove the accessory outlet from the back panel and enlarge the opening to accomodate a rocker style on/off switch.

Don't forget to install a fuse holder and a proper 3 wire power cord.

smbrown 30th January 2008 01:27 PM

Thanks for the recommendations, Eli. I've pretty much gutted the amp at this point and about ready to begin rebuilding. I discovered last night that a Hammond 10H, 65ma choke will fit under the hood (with a shoe horn), and that gave me an idea to make the PS a choke input filter, thus lowering the voltage to the output tubes by around 70v. Appreciate any thoughts you might have on that approach. I'm going to have to begin the search now for a sister unit so I can do this in stereo, but these little guys seem to be commanding higher and higher prices on ebay these days.

Eli Duttman 31st January 2008 01:44 AM

Steve,

That choke has too low a current rating for this project, with a (sic) cap. I/P filter. A "beefy" inductor is needed for choke I/P service, as takes a royal beating.

The gyrator has the advantage, for this project, of not generating a magnetic field. Locating a choke so its field does not interact with the 2 trafos on the A7 chassis is difficult.


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