filling in the low end, 6bq5 12ax7 circuitry

[trigger]

i salvaged this amp out of an old radio i pulled from the trash, finally got it fired up properly, it sounds amazing now after running an hour, the low end is faint under 80hz.

any thing i can do to improve the low end besides new OT's ?

i dont have a budget to work with, but i do have many vintage radio parts on available.
well all i need do now is get it all pretty with gold terminals and a polished wood case. there is a pre that i also salvaged off the radio i plan to make a small unit that uses the power supply off the power amp. fun stuff! thanks guys for the advices!

 

[kavermei]

Schematic diagram please!

 

[rotaspec]

I'd certainly look at replacing the bypass capacitor between the 6BQ5s (black electrolytic). Probably the two other coupling capacitors too.
However, with unknown OTs you are in the lap of the gods as to how far down they will reach.

 

[TheGimp]

Another thing to consider is replacing the PS filter caps. They are AL Electrolytic and have probably lossed considerable capacitance over time. That may effect your low end, and is worth replacing even if it isn't.

 

[Soonerorlater]

Hi Trigger,
Is the third picture a picture of the amp you are taking parts out of?

( It has an mains earth crimp dangling over a valve base)

Best Regards
Bill

 

[Soonerorlater]

Hi trigger,
Rotated the last picture 180 degrees and now I can see your work before and after.

Best Regards
Bill

 

[rotaspec]

Coupling caps should be 0.1uF or greater, 400v or higher rated.
Bypass should be 220uF or greater, 25v or higher rated. Buy a new one for this position.

 

[Bob W7ETA]

I have an integrated UA-1 Heathkit using the same rectifier tube. I installed 120uf 450V caps in the power supply.

For coupling caps I used .22uF K40-Y 1000V Russian PIO. 220uf 50V Nichicon bypass caps. 22uf 100V coupling cap between the 6AN7 (pentode-triode) and the EL84s. I tried doubling the coupling caps to .44uF/ It either didn't make a noticable difference or I didn' leave them in long enough to break in.

I replaced all of the resistors with carbon film except for resistors directly in the audio path, carbon comp. I had a Russian PIO of the correct value for the feed back circuit, so I installed that.

Someone pointed out that replacing the .5watt resistors with 1 watt ones wouldn't cost a lot more, and that they should be quieter.

After rebuilding, the amp sounded AMAZING! Base improves, plus the pace of music improved. The last thing I tried was Belden cable from Parts Express, the RED!!! stuff with just a three conductor plug from Home Depot, another striking improvement!!

I discovered that the black plate square getter white lettering RCA 6AN7 were pretty remarkable. If you find a schematic using a pentode triode combo tube with single EL84 you can take the 12AX7 out and just add a second socket.

I mention that because really good sounding 12AX7s are hard to find, and are a lot more expensive than the RCA 6AN7 or other similar combo 9 pin tubes.

I recently traded some Raytheon black plate two mica NOS 5751 for a pair of Brimar box plate CV4004--finally found a 12AX7 that I really like. Since I like that tube so much, I plan to experiment using a 9 pin pentode to drive it in place of the pentode-triode tubes I like. tubes like D3A pentode are still pretty CHEAP, as are a lot of other very good pentodes.

Enjoy your new iron.

Bob

 

[trigger]

well very sweet this amp has come alive, i replaced the caps that rota and bob mentioned, and thanks guys! nailed it!!!

the amp was missing the low end, i decided to increase the impedance load on the output, to 4 ohm, then 2, and well the problem came to light, a 6bq5 started glowing red, witch showed me the caps were the culprit.
anyways i replaced the two 1mfd (color banded) with two nice fat .39 goodalls, and that black coupling cap with a new one from radioshack, and boom..
the low end is very responsive and deep now, the highs light right up when the volume is at proper max, the amp also showed much more dynamics after the new caps installed.

thank you much!!!

i just need a sweet 12ax7 to rock, i have 3 tungsol 12at7's that were pulled from an organ and a few pair of 12au7's trade anyone?

i will replace the resistors and maybe the large caps on the power side, case the amp in wood and call it a day, half done and sounding sweet...

 

[Bob W7ETA]

Here you go:
0,22uF +/-10% 1000V PIO Capacitors K40Y-9 NOS Lot of 8 - eBay (item 260529442556 end time Jan-26-10 15:29:47 PST)

Fantastic caps for less than $2 each!

 

[trigger]

old caps

thanks for the suggestion, i am tapped out on a budget for any projects this season, i do live in a county that should be considered its self, a junkyard. many old parts available to me, and thus, i have a few questions about old caps...

is there a way to test caps for audio quality, besides installing and listing to them?

i pulled these goodall caps out of an organ, pictured below,
are the long white ones be any better than the ones i installed? a guess?
long white 200vdc, the ones i installed are 150vdc


pic1 a tube................ pic2 white caps............. pic 3 installed caps

 

[Bob W7ETA]

Caps you can afford always sound better than ones you can't afford.

If you got em, just gotta try 'em.

If you have extra parts, maybe you could swap extra parts with someone who has extra Russian PIOs? Can't beat 'em for their cost.

Best from tucson
Bob

 

[rotaspec]

The only concern I have is that maybe the 150v rating might be too low on the ones you installed. Might pay to check the DC voltage across them, just to be sure.