Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Joe6v6 on February 13, 2010, 07:41:25 pm
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Here is a layout that I put together for 6v6's. I modified a layout that normally uses kt66's so I havent made a schematic yet. My question is will the secondary voltages of the indicated transformer produce too much voltage for 6v6's ? I have this transformer on hand and want to use it if I can. It does not have a 5v tap or I would use a 5y3 rectifier & probably be ok. If I do use that sag resistor is 5 watt OK ? How much voltage, if any, can I expect that resistor to drop? .. .. Joe
(http://img.photobucket.com/albums/v234/joe6v6/th_Highway6v6.jpg) (http://smg.photobucket.com/albums/v234/joe6v6/?action=view¤t=Highway6v6.jpg)
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Some would say to put the 68k grid stop right at the socket of the first stage, rather than the input jack...
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My question is will the secondary voltages of the indicated transformer produce too much voltage for 6v6's ?
Not if you bias it cool.
If I do use that sag resistor is 5 watt OK How much voltage, if any, can I expect that resistor to drop?
That depends on how hot you run the 6V6s. A typical 6V6 amp might idle at 25ma per 6V6, then figure 50ma thru that 250Ω resistor. That's a 12.5v drop and the resistor will dissipate 0.625w, so a 5 watt resistor would be OK.
I just glanced at your layout, but two things jumped out at me.
1. The source feed for the bias circuit needs to be connected to a PT HT winding, not the cathodes of your 1N4007 rectifiers.
2. You need a filter cap at the junction of the PI plate resistors. (just south of that pink 100pF cap)
To me, it just seems backwards to do a layout before drawing a schematic.
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Some would say to put the 68k grid stop right at the socket of the first stage, rather than the input jack...
I know techinacly thats where thay say to put it, ive done it both ways & never noticed a difference & latly I just leave it out.
To me, it just seems backwards to do a layout before drawing a schematic.
I have a schematic I made but its witk KT66's and a gz34 rectifier but I havent got around to changing it yet. . Good catch on the bias feed, I may have gone all they way with that one. .
You need a filter cap at the junction of the PI plate resistors
Should there be one there other than the one at the top of the 10K droping resistor ?
Sluckey - Do you think I should leave that sag resistor at 250R or change its value or leave it out? Ive never put an amp together with one and I dont know what to expect.... Tx : Joe
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Looks like that is based on the Route 66.
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Should there be one there other than the one at the top of the 10K droping resistor ?
I think so. The B+ for the PI will be much smoother with the additional cap. Also, without the additional cap, that 10K dropping resistor actually becomes part of the plate load resistors. I'm not sure if that will have a noticeable effect on the two out of phase signals. Are you really sure you want to use parallel B+ nodes rather than the more traditional series nodes?
Sluckey - Do you think I should leave that sag resistor at 250R or change its value or leave it out?
I'd leave it there. Doing so will make it easy to adjust the B+ up/down if needed. You can easily and neatly replace the resistor with a jumper wire if you decide to remove it later on.
Take a look at the attached pic and I think it will make sense to you to add the cap to the PI node. And if you're cloning the amp that topbrent posted, why not just copy that power supply?
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I wouldnt bother too much about Sag and trying to capture it. I have built alot of amps and i only had one an old champ that actually you could hear a noticable difference in. For me anything above a single 6V6 tube with a 5Y3 just gets lost somehow. I may be wrong as well but just my opinion /
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I wouldnt bother too much about Sag and trying to capture it.
Im not wanting to use it as much for sag as I am wanting to drop the B+ voltage a little.
This is the schematic I was initally looking at. Its very similar to the one Brent posted except for the power supply. It seemed to match more closely with gut pictures of a root66.
Is this revised layout a better way to go as far as the power supply?
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This is the schematic I was initally looking at.
I guess I didn't know what I was talking about. :embarrassed:
Your original layout matches your schematic. You don't need the extra cap after all. Sorry for the confusion.
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I've built a similar design with a very similar layout with no trouble- look good to me!
The only thing I did differently was to mount the final filter cap near the pentode it's serving and ground it on the input side of the amp. It's doubtful you'll have any trouble the way you have it but I thought I'd mention it.
jamie
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Your original layout matches your schematic. You don't need the extra cap after all. Sorry for the confusion.
But would the additional cap be a better way of doing it? Seems like most amps are done that way.
I dont mind adding it, if its in a can it wont change the board any.
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But would the additional cap be a better way of doing it?
My Vulcan side says yes. My mojo side doesn't know. The absent cap may be part of the 666 magic. :grin:
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I swore there was a leak hifi amp that had that arrangement with the shared 10k but I've been unable to find it.
here's one example but I thought there was one that more closely resembled the amp you're building.
http://ampslab.com/SCHEMATICS/LeakTL10.jpg (http://ampslab.com/SCHEMATICS/LeakTL10.jpg)
I guess it was an era when caps were a lot more expensive!
jamie
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> ive done it both ways & never noticed a difference
The difference is inaudible.
> & latly I just leave it out.
That works fine UNTIL you play next to powerful radio transmitters. Maybe you never do. Or maybe your next big gig will be a truckstop full of CB radios. 99% of the time you don't need it, but for that 1% it is worth much more than 12 cents to not have "*#* 10-4! *+#! COME BACK BIG BUDDY!" accompanying your solo.
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it is worth much more than 12 cents to not have "*#* 10-4! *+#! COME BACK BIG BUDDY!" accompanying your solo.
Point well taken. Those are words I never hoped to hear again under any conditions.
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> ive done it both ways & never noticed a difference
The difference is inaudible.
> & latly I just leave it out.
That works fine UNTIL you play next to powerful radio transmitters. Maybe you never do. Or maybe your next big gig will be a truckstop full of CB radios. 99% of the time you don't need it, but for that 1% it is worth much more than 12 cents to not have "*#* 10-4! *+#! COME BACK BIG BUDDY!" accompanying your solo.
Very funny!
Unfortunately, looking back through this thread I'm confused about which 12 cent part we are talking about.
Chip
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which 12 cent part we are talking about.
Grid stopper between input jack and first preamp tube.
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> which 12 cent part
33K-68K from jack to grid.
With the internal capacitance of a 12AX7 grid, this makes a high-cut with negligible effect at 20KHz but a good 10:1 reduction on the AM radio band, more for most other radio services.
The classic Fender value is two 68K in parallel, effectively 34K. Using one 68K gives a just-inaudible difference at the top of the guitar zone. If you obsess about one-dB difference beyond what most pickups deliver, 33K is nearest standard value to Fender's 2*68K. But going 2:1 either way (to use parts in hand) is not a problem.
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Joe, have you built this yet?
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I've seen that in some Mesa Boogie amps
instead of the 68k resistor
they use a ferrite bead
I think this gives the advantage of the absence of the resistor (nothing is loss)
and prevent RF incoming
but this solution is uncommon
There is a particular reason ?
Ferrite beads are not very difficult to be find at radio amateur markets
Kagliostro
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One other thing:100R screen resistors don't cut it for 6V6's.I would use at least 470 ohm.
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Make sure you use a 25watt resistor if you go with a 250R SAG. The most i've ever used is 150R and I had to parallel two 10 watt resistors for that. One 10 watt gave off too much heat. joe
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Joe, have you built this yet?
No not yet, but I have the board built & started the chassie and I have all the other parts. I got side tracked with other stuff( Buildin a 302 & fixin the bathroom.
One other thing:100R screen resistors don't cut it for 6V6's.I would use at least 470 ohm
. Yea, im not sure how those 100r got put there, 470R it shall be.
Make sure you use a 25watt resistor if you go with a 250R SAG.One 10 watt gave off too much heat
Ive heard of others using 5watt without problems in a similar set up, someone said that resistor would disipate less than one watt?