AA763 Champ home brew B+ ?

[catnine]

I built this amp with all the same values off the fender field guide other than I did use higher voltage caps .

I used weber PT and OT and have a NOS 5Y3 and RCA 6V6GTA .

Right now I have a B+ of 383 VDC and a Plate of 370 VDC . The 6V6 is set at 12.49 IPD using a 680 ohm cathode resister this is when the line is 118 VAC when it's 119.5 VAC the IPD is 12.66 . the cathode voltage is 24.6

The PT has a choice of two secondary taps . 330-0-330 or 300-0-300 . I used the 330 . the original champs ran about 350 of the plates .

Here is the question . I hear you will not get full power out of a 6V6 at high plate voltages and that using a higher resistance cathode resister causes the 6V6 to break up sooner . My amp seems pretty clean and begins to break up at 6 1/2 on the volume and it is loud .

If i use the lower 300 taps and then in doing so I will probably end up being able to use the stock value cathode resister of 470 ohms . Will this render an cleaner clearer tone by dropping the B+ and keep the tube within the tubes class A bias specs or will it make much difference at all . The diagrams do say voltages + or - 20 % .

[PRNDL]

This is a somewhat complicated answer, and you may want to review some tube amp design sites.

IIRC, the general idea for headroom is to design the amp so that the plate voltage of the tubes is 2/3 of the B+. In that way, the cathode voltage has the maximum swing before hitting the B+ rail, which causes clipping and distortion. Lowering the B+ voltage has an effect on the entire amp - both preamp and output.

The simple answer to your question, however, is to simply try it for yourself and see what it sounds like. It's a fairly simple mod - one wire.

[catnine]

This is a somewhat complicated answer, and you may want to review some tube amp design sites.

IIRC, the general idea for headroom is to design the amp so that the plate voltage of the tubes is 2/3 of the B+. In that way, the cathode voltage has the maximum swing before hitting the B+ rail, which causes clipping and distortion. Lowering the B+ voltage has an effect on the entire amp - both preamp and output.

The simple answer to your question, however, is to simply try it for yourself and see what it sounds like. It's a fairly simple mod - one wire.

Well actually to use the 300 VAC tap I would need to remover the two 330 red lead from the 5Y3 socket and solder the 300 VAC leads in their place and drop to value of the bias resister to get the IPD back up . To go 2/3 of the B+ means 255 VDC plates with a B+ of 383 VDC and if I dropped the B+ down 30 VDC I am far below a champ voltage all across the board then it's no longer a champ .

One issue this PT might have is even though it is a champ replacement PT offed by Weber it has a 100 mA rating where the other champ PT's are 70 mA which allows this PT to keep VDC higher .

I guess I will think about going to the 300 VAC taps or just leave it as is because a high B+ seems to be an issue with most all of the fender SF or later champs where it seems fender increased the PT VAC about the time the bronco came out , you see this footnote on the fender field guide schematics on the champ where the foot note is hand written and is the same number as the bronco and the bronce has a higher VDC spec.

The voltages I have now are a bit lower than they were with my 73 SF Champ with the same 680 ohm cathode resister.

[tubeswell]

Well actually to use the 300 VAC tap I would need to remover the two 330 red lead from the 5Y3 socket and solder the 300 VAC leads in their place and drop to value of the bias resister to get the IPD back up . To go 2/3 of the B+ means 255 VDC plates with a B+ of 383 VDC and if I dropped the B+ down 30 VDC I am far below a champ voltage all across the board then it's no longer a champ .

You can always use the 300 VAC tap on the PT but use a 5R4 instead of a 5Y3GT, and then maybe lower the supply resistor to keep the pre-amp voltages higher if need be.

[catnine]

You can always use the 300 VAC tap on the PT but use a 5R4 instead of a 5Y3GT, and then maybe lower the supply resistor to keep the pre-amp voltages higher if need be.

I could but this would put me right back very close to where I am now except that the current draw on the 5R4 just might allow the voltage to drop more than the 5Y3 with the 330 VAC tap .

It;s difficult to tell unless I do it because I can only calculate that I will have a 30 VDC B+ drop it might be more or less than that because what I don;t know is there is a 100mA rating on the 330 VAC taps but I don't know what the rating will be on the 300 VAC taps , this is not specified on the weber PT spec sheet. I can assume the mA rating would increase using the 300 VAC taps on this PT. Right now the preamp pin 1 is at 240VDC #3 is 1.7 #6 238 and #8 1.81 VDC. Over all I am 10 % above what the champ schematic calls for and it shows a line VAC of 117 VAC and my line is either 118 or 119.5 VAC depending on the time and day and I have seen it as low as 117.2 like last night . I live in an old apt with old wiring where 120 VAC is almost a once a year event.

First I want to see what the IPD works out to be with a JJ 6V6s before I change anything . Each tube is different and the JJ might draw more than the RCA 6V6 and this might pull the plate down some but I don't expect much however since the JJ 6V6 s does offer a tone closer to a 6L6 then I might be able to lower the bais resister value and this will lower the B+ and Plate and I will still retain the low end and headroom I have now.

When I first built this amp I used a 47 UF for the 1st filter cap and a 22 uf for the 2nd and 3rd and had the 1 K cathode resister IPD at 10.25 watts and the amp sounds real good to me , then I was told that a 47uf 1st filter cap was too much for a 5Y3 so I changed it to a 30 uf and I lost some bottom end and this brought on the beginning of the icepick highs . A JJ EC83 fixed that . I then changed the 1K to a 715 K using three 5 watters in series which is all I had and the amp sounded better then I went with a value of one 5 watt at 750 OHM and lost the clarity so in went the 680 ohm and this brough that back.

Seems the slightest change in a value makes for unexpected results so now I am a bit gun shy in fear of making things worse rather than better.

This amp chassis is an Allen amps tweed style but has the on/off and a standby and a place for a fender slide switch and the vol/treb/bass BF setup . the best would be to use the standoff as a DPDT and wire the PT secondaries to that and have a 1K 5 watt pot for the cathode value then I can plug and play then choose the best value for the sound I like then wire in the values I find best.

[tubeswell]

A 5Y3GT will still work okay with a 40uF (or even 47uF) reservoir (even tho' the datasheets say 16uF or 32uF depending on which sheets you look at), it just won't last for ever. I did a 5F2A with 40uF and it still works fine after nearly 2 years. You can use an even bigger reservoir cap if you put fairly robust series limiting resistors between the HT and the rectifier's anodes. (You probably should have some other protection - like 1000V diodes - there as well in case the rectifier shorts).

The current draw on the 300V tap won't be that much different to the other tap. The thing that will affect the PT voltage more is the current draw on the heater winding. As long as the heater winding is fairly conservatively rated for the total filament load, the HT voltage should hold up.

[catnine]

A 5Y3GT will still work okay with a 40uF (or even 47uF) reservoir (even tho' the datasheets say 16uF or 32uF depending on which sheets you look at), it just won't last for ever. I did a 5F2A with 40uF and it still works fine after nearly 2 years. You can use an even bigger reservoir cap if you put fairly robust series limiting resistors between the HT and the rectifier's anodes. (You probably should have some other protection - like 1000V diodes - there as well in case the rectifier shorts).

The current draw on the 300V tap won't be that much different to the other tap. The thing that will affect the PT voltage more is the current draw on the heater winding. As long as the heater winding is fairly conservatively rated for the total filament load, the HT voltage should hold up.

I am still trying to understand what the advantage is of useing the 300 VAC tap instead of the 330 VAC tap. Is it just to be able to bias the 6V6 for a better swing before clipping and if so will this render an end result of the cleaner tone. I read having higher voltages on the preamp plates contributes to headroom and this seems to be why you brought up adusting or lowering the preamp dropping resister value .

The reason I even question this is many AA763 champ onwers are faced with a stock original PT and they face the same issue of higher voltages across to entire amp , soom raise the cathode resister value so the 6V6 is not about 14 watts , some add zener to drop the B+ , some just let the amp IPD with the stock 470 ohm cathode resister and let the 6V6 idle well past 14 watts some at 16 or 17 watts and like the early breakup . I don't like the crunch sound .

When I got a 73 SF champ I changed the electrolytic caps , all of them and used a higher voltage rating but stayed with the same uf values this includes the cathode bypass cap , mine is 160 volt rated .

When I got the 73 SF champ I took readings and had 23.1 across a drifted cathode resister which was 506ohm , it had 386 plate VDC and 400 VDC B+ .

I changed the dropping resisters to 3 watt at the same 1k and 10k values and was told to use a 1k ohm cathode resister and this gave me 27.1 cathode VDC 397 plate and 405 B+ yet the amp sounded clear and clean and responsive especially at low volume levels . The cap can in the original was a 40/20/20 and I replaced it with the same .

See what I don;t want is a lot of breakup at low levels or grind at higher levels and less bottom and my main fear is I can go to a lower cathode resister from say the 680 in there now to a 600 ohm and probably end up at 14 watts IPD and a lower Plate than the 370 VDC I have now and a lower 383 B+ I have now . But this seems a question that I may end up with more breakup sooner . I look at a fender bronco schem and it shows a 365 VAC PT and yet at the plate it shows 342 VDC which is lower than the champ with a 320 PT and a 350VDC plate all other things remain the same so this schem can't be right . I also see around the time the bronce came out the champs say later used the same PT and the bronco which is the higher voltage PT with the same number hand wrttien on the champ schematic as used in the bronco.

I built a few kits of the older champs with lower PT voltages and a few with the tone like the Princeton 5F2A using hammond Pt's 250 VAC but was not impressed with the sound at all .

I did a search and saw a post by you with a fellow asking about a dual purpose champ AA763 and a early champ making it switchable . you recommended a 5F2A and not to worry about the high plate , this is not a slam but a question about champs running higher voltages because there are so very many opinions on this subject and many different ideas and views and remedies that I get lost in the mix.

Here's my deal , my champ SF home brew is only 10 % over the voltage values and fender allows + or - 20% as the norm , my glitch is that the preamp tube is at 20 % power tube and B+ 10 % over and the cathode VDC is 5.6 VDC higher than the champs 19 VDC . So am I that far off spec to really change the champ sound much .

Each princeton and champ from the first ones to the SF all have different changes such as the tone pot values and the way the tone stack is wired to Linear taper pots to audio taper pots , the 5F2A uses the same setup as the 6G2 yet uses linear tapper 1Meg pots and the 6G2 PP princeton uses audio tapper . It get confusing and I have not ever played a old fender champ or princeton so I have no idea how they sound or breakup . I don't like the way the SF tone stack works it is a loss I know this because in a 6G2 build I have I tried both and the vol/tone won hands down and no NFB rendered the best sound for my taste but to make this change to my AA763 build SE may be another story all together.

[tubeswell]

Oh I see now - well if you want more headroom, then you can indeed use a higher output tube plate voltage (but make sure you bias the tube appropriately). The JJ6V6s is a good tube for higher plate voltages and can easily run at 14W (or even 16W). The trade off of re-biasing the tube with a higher cathode voltage, is that you will lose some gain.

Or otherwise you could go to a GZ34 (or go to diode rectification), and put a bigger output tube in (like a 6L6GB/5881, which you should be able to do with a 100mA HT - and run it at around 410V-450V - and it will only suck and extra 450mA from the heater winding), but you will have to re-bias and change the primary load through the OT. The advantage of a bigger tube if you want more headroom, is that it takes more signal voltage in the control grid for it to clip (so your driver tube won't clip the output tube control grid). But make sure your OT is up to the task (in terms of ability to switch load resistances and the size/power handling ability)

You can also lower the signal driven into the output tube (if you want to keep the 6V6 in there) by playing with the gain in the preamp stages, by variously:

Going to lower gain pre-amp tube(s) - esp the driver stage; and/or

Lowering the plate resistor values (but don't go below about 47k with a 12AX7); and/or

Increasing the cathode resistor values (try anything up to 4k7); and/or

keeping fairly large bypass cap values (about 22-25uF or more); and/or

Omitting one or more of the bypass caps; and/or

Putting in some local NFB resistors (from after the coupling cap back to before the signal grid of the same stage); and/or

Lowering the size of the global NFB resistor

But even if you do these things, there is a limit to the bass the your OT can carry, and you only get so much bass you can get with a smaller OT before it saturates.

Sorry if this doesn't help you much - it just my 2 cents worth. :-)

[catnine]

Oh I see now - well if you want more headroom, then you can indeed use a higher output tube plate voltage (but make sure you bias the tube appropriately). The JJ6V6s is a good tube for higher plate voltages and can easily run at 14W (or even 16W). The trade off of re-biasing the tube with a higher cathode voltage, is that you will lose some gain.

Or otherwise you could go to a GZ34 (or go to diode rectification), and put a bigger output tube in (like a 6L6GB/5881, which you should be able to do with a 100mA HT - and run it at around 410V-450V - and it will only suck and extra 450mA from the heater winding), but you will have to re-bias and change the primary load through the OT. The advantage of a bigger tube if you want more headroom, is that it takes more signal voltage in the control grid for it to clip (so your driver tube won't clip the output tube control grid). But make sure your OT is up to the task (in terms of ability to switch load resistances and the size/power handling ability)

You can also lower the signal driven into the output tube (if you want to keep the 6V6 in there) by playing with the gain in the preamp stages, by variously:

Going to lower gain pre-amp tube(s) - esp the driver stage; and/or

Lowering the plate resistor values (but don't go below about 47k with a 12AX7); and/or

Increasing the cathode resistor values (try anything up to 4k7); and/or

keeping fairly large bypass cap values (about 22-25uF or more); and/or

Omitting one or more of the bypass caps; and/or

Putting in some local NFB resistors (from after the coupling cap back to before the signal grid of the same stage); and/or

Lowering the size of the global NFB resistor

But even if you do these things, there is a limit to the bass the your OT can carry, and you only get so much bass you can get with a smaller OT before it saturates.

Sorry if this doesn't help you much - it just my 2 cents worth. :-)

It does help , thank you , perhaps I did not make it clear what I was trying to do in the first place .

I did add a 250K pot to a slide switch to be able to lower the NFB and it opens the amp up and a flip of the switch and it's back to the stock value NFB . That's to only mode I did , tried the switch to break the mid range 15K resister from ground on the tone stack , hated that sound .

In the back of my head I keep thinking that using a lower B+ will render better results but from what I've read this will make the amp more gainy .

I used the 25 uf bypass caps on the preamp and wired this amp right off the fender field guide schematics with all the proper vaules all I had to do was bias the 6V6 lower and did so with a 680 ohm cathode resister putting the 6V6 at 12.50 IPD .

I did a build using two 6v6's in parallel using a allen amps OT to 8 ohms and with this I could pull the two 6V6's and install one 6L6 but the tone was pretty close a bit more tight and clear but I was not blown away by it.

I play blues but not the sort with a lot of breakup and distortion and with this amp if I want a bit of breakup I use a rocktron austin gold OD pedal and it does the job fine.

However I could live with breakup at a lower level because this amp is quite loud at 6 on the volume , not a bedroom practice amp on 6 by any means . I have it in a larger cab than a champ and I have a weber ceramic sig 10S in it so it does hold the bottom solid ,well as good as a champ can produce bottom end . I should have gone with a weber alnico sig 10 and the amp would probably have the lower level breakup and sound just a bit looser and warmer. Maybe when funds allow this will be in the cards.

I am just trying to reach a happy medium with breakup and retain the headroom and bottom but there is always a tradeoff .

I have a build With the tone stack of a 6G2 princeton and a 5E3 power section that is adjustable fixed bias running a 12Ay7 V-1 with a 12AX7 V-2. V-2 is like the 5E3 with the second gain and PI . I used allen amps champ PT and allen amps deluxe OT and JJ 6V6's and I can switch out the 25 uf bypass cap on V-2 gains stage and bring in the 6G2 NFB loop but still use the 12AY7 preamp tube because with a 12AX7 as called for by 6G2 princeton spec it is to gainy for my tastes .

So with that said I can use that for loud clean and the champ I can use for more breakup blues tone if I can hit that happy medium I expressed above.

Seems to me people with SF champs either let the 6V6 run hot of add zeners to lower B+ or build with a lower rated secondary PT . My chassis I used for this champ build is a tweed style allen amps used to sell and it came with a fender PT cutout so I went with a weber PT since they are good and not breaking the bank .

Maybe once the JJ 6V6 blows in this week and the two 300 ohm 5 watts and one 500 ohm 5 watt resisters I will try the 300 VAC taps and see what this does because it does seem well known you don't know until you try . I just changed from a 750 ohm to a 680 ohm cathode resister early this week and that made a big change in the amp , sounds better to me now. still below 14 IPD .

[tubeswell]

If you are thinking about a syrupy thicker sort of sound, maybe you could try lowering the B+ (to say around 320-340 on the plate) and biasing for hotter current (with a lower cathode resistor - like around 200-220R), then upping the pre-amp voltages with a lower supply/dropping resistor (but really the best way to get this sort of sound cleaner at louder volumes is with a low-powered PP amp I reckon. SE gueetar amps are just plain distorty amps by nature).

[catnine]

If you are thinking about a syrupy thicker sort of sound, maybe you could try lowering the B+ (to say around 320-340 on the plate) and biasing for hotter current (with a lower cathode resistor - like around 200-220R), then upping the pre-amp voltages with a lower supply/dropping resistor (but really the best way to get this sort of sound cleaner at louder volumes is with a low-powered PP amp I reckon. SE gueetar amps are just plain distorty amps by nature).

Yes I know . Champs were a cheap practice amp and many are still around and popular because of their tone and low level volume but were never designed to be cranked full out and that at their time was their short coming as were many older tweed amps of low wattage and simple circuits.

In the early 60's I had a harmony combo with a 12 , I don;t know if it was push pull but I think it was SE and it broke up at very low levels and it was at the time a real dissappointment to me.

But this is what champs were , practice amps designed to be at 3 or 4 on the volume at best .

Once I get the new tube and check bias I may be able to drop the cathode to 600 ohm and run it closer to 14 watts , if that makes it worse then I will put the 680 back in and live with it because over all it is not a bad sounding amp , it does sound like SE amp just more headroom and less breakup than a champ usually would be and that in a way is a success for me.

The SF were a l lot cleaner than the older champs and this was fenders goal at the time and that was success on their part . I just have to ignore the schematic voltages because I have yet to see a champ at those voltages .

[tubeswell]

I just have to ignore the schematic voltages because I have yet to see a champ at those voltages .

Know what you're saying, the 5F2A I was talking about sounds better with the plate around 360 and the 40uF at the first filter- it is too spongy otherwise. But if you lift the NFB and dime it is does a real good stairway solo. I built it to also run on a 6L6 with a GZ34 (as well as a 6V6 and 5Y3GT) (PT is 120mA 318-0-318), and sold it to a guy who plays quite a bit of jazz and he put a ECC823 or a ECC832 (can't remember which, but with the low-gain triode in V1A) and loves it.

[catnine]

Know what you're saying, the 5F2A I was talking about sounds better with the plate around 360 and the 40uF at the first filter- it is too spongy otherwise. But if you lift the NFB and dime it is does a real good stairway solo. I built it to also run on a 6L6 with a GZ34 (as well as a 6V6 and 5Y3GT) (PT is 120mA 318-0-318), and sold it to a guy who plays quite a bit of jazz and he put a ECC823 or a ECC832 (can't remember which, but with the low-gain triode in V1A) and loves it.

My JJ 6V6s blew in today so I was able to check the bias with that and the B+ dropped a bit about 7 VDC but the catho=de VDC raised about 1 VDC . I have not had a chance to play it today but that's what I had in my 73 SF champ so we'll see what gives. I may be just the ticket . I decided to leave the 330 VAC tap alone and just try this and if it is not better then a weber alnico sig 10 S manybe my next step if funds allow. Each 6V6 is different even the 12AX7's by brand make a change . I will find the sweet spot somehow and I don't expect a lot just hope the new JJ tames the amp some what .