5f6a field trial

[lmolter]

I brought the amp home today. No luck finding a RI or anything, so I plan to exorcise the chassis from the cabinet and give it the ol' eyeball treatment.

A couple of things in the last posts caught my eye. I'm running 35ma at 470v through the 5881's. This is about 70% of the rated 23W dissipation. Is this considered moderate or cold?

The filter cap in the first stage (assuming what is meant is the first preamp stage) is 8uf as per the original schematic. All other caps (4) are 20uf. No bleeders. And the choke is a small 3H choke, though it is stock.

I don't know the vintage of the 12AY7. I had a few lying around, and although I bought all new tubes for this build, it may have been swapped out for something else during testing. And what about swapping it out for an AX7 just for testing purposes? I should get the amp to break up sooner, right? Will the tone change at all?

I am also planning to put in a 30V Zener to lower the B+. Should I even bother? Will the tone change that much?

After all is said and done, I should be an expert on this thing in no time <g>.

L

[txstrat]

If you have a 12AY7 in the preamp, it would at least explain the amp running clean on 10.
A 12AY7 has an amplification factor of 40 while a 12AX7 has an amplification factor of 100.
With a 12AX7 the amp should sound VERY different due to more gain.
Try and see what happens.

[tubeswell]

Also remember that when these amps were first being made, the goal was to build something that could provide a nice clean, lean and mean sound. So by '59 they were coming out with LTP inverters, fixed bias, beefier rectifiers and higher voltages on the output tubes than previously. So if you're using a 12AY7 in V1 to boot, you should have a nice cleanish sounding amp.

If you want it to get a bit dirtier, try lowering the B+ with a lower-powered recitifier tube (have you got a 3A 5V winding? then try a 5U4) and rebias for more current. (Or even a 5V4 and a pair of 6V6s and rebias etc) - and you may need to tweek the pre-amp supply rail.

Another thought I had was, is your LTP putting out the maximum signal you can wring out of it? The lower the tail voltage, the more unbalanced it will be, but the more gain you will be able to get out of it (esp if you can keep the LTP plates comparatively high) for driving the output valve grids. This will help it on the way to being more distorted. (Read up the valve wizard site article on LTPs http://www.freewebs.com/valvewizard/acltp.html)

[lmolter]

Hey, tubeswell, part of the problem is that I think the amp sounds just fine. It's nice and clean and loud (DID I SAY 'LOUD'?). The highs are bell-like, not sharp or tinny. Just what are the folks I lent it to expecting? Alas, if I were a good player, I might have an idea. But I'm not. I'm just the technician/builder/hacker/fiddle-player.

From a troubleshooting standpoint, I wish someone would post a schematic with ac signal levels from input to output. Starting with 10mV @1KHz at the input, show me what to expect as I work my way through the circuit. That would be very handy. Of course, I've done that myself, but just to see if I was introducing hum or distortion along the way -- but I have no idea what signal levels to expect. Is the LTPS putting out? I have no idea. The two outputs were clean and 180 degrees out of phase. To me, it was working fine.

The other problem I have is that the amp is too bloody loud to fully test at high volume. I put in an AX7 in V1, cranked it up to 8 and the dog started the most gutteral howling I have ever heard. (Might have been my playing). But I can't tell you if the amp did any growling. Still sounded good. What's it gonna take to overdrive this beast? An ear bleed?

So... maybe the amp is just sounding like it would have if the year was 1959. But just to ease my mind, I'll head over to the local music store this weekend and beg someone to play the '59 RI Ltd. Maybe I can hear what the highs are supposed to sound like. And should I even bother with the Orange Drop upgrade and the B-plus-lowering Zener mod?

Not discouraged yet.

Larry

[Diablo]

I'm running 35ma at 470v through the 5881's. This is about 70% of the rated 23W dissipation. Is this considered moderate or cold?

The filter cap in the first stage (assuming what is meant is the first preamp stage) is 8uf as per the original schematic. All other caps (4) are 20uf. No bleeders. And the choke is a small 3H choke, though it is stock.

I don't know the vintage of the 12AY7. I had a few lying around, and although I bought all new tubes for this build, it may have been swapped out for something else during testing. And what about swapping it out for an AX7 just for testing purposes? I should get the amp to break up sooner, right? Will the tone change at all?
L

My 5F6A is biased about the same as yours, 33 and 36mA @ 462 volts.
I'm using a little larger capacitor for the main filter than you. You have 40mfd, and I have two 100 mfd/450V in series (50mfd effective) with bleed resistors. The rest of my filter caps are the same as yours. My amp does break up using a 12AY7 for the first tube. However, the breakup really depends on the signal strength of the guitar. Single coil pickups are a weak signal compared to a humbucker, so with the humbucker it breaks up at a lot lower volume setting. As far as the break up issue, make sure your input jacks are wired correctly. Another fellow on this forum had that problem. Yes, you can try some different tubes too. Maybe you just have a weak preamp tube. Try swapping some in, one at a time. The best thing I can recommend is to haul your amp down to a music store to compare to a 59 RI. Have their local Jimmy fast-fingers test drive both amps while you listen.

[Satamax]

Hey, tubeswell, part of the problem is that I think the amp sounds just fine. It's nice and clean and loud (DID I SAY 'LOUD'?). The highs are bell-like, not sharp or tinny. Just what are the folks I lent it to expecting? Alas, if I were a good player, I might have an idea. But I'm not. I'm just the technician/builder/hacker/fiddle-player.

From a troubleshooting standpoint, I wish someone would post a schematic with ac signal levels from input to output. Starting with 10mV @1KHz at the input, show me what to expect as I work my way through the circuit. That would be very handy. Of course, I've done that myself, but just to see if I was introducing hum or distortion along the way -- but I have no idea what signal levels to expect. Is the LTPS putting out? I have no idea. The two outputs were clean and 180 degrees out of phase. To me, it was working fine.

The other problem I have is that the amp is too bloody loud to fully test at high volume. I put in an AX7 in V1, cranked it up to 8 and the dog started the most gutteral howling I have ever heard. (Might have been my playing). But I can't tell you if the amp did any growling. Still sounded good. What's it gonna take to overdrive this beast? An ear bleed?

So... maybe the amp is just sounding like it would have if the year was 1959. But just to ease my mind, I'll head over to the local music store this weekend and beg someone to play the '59 RI Ltd. Maybe I can hear what the highs are supposed to sound like. And should I even bother with the Orange Drop upgrade and the B-plus-lowering Zener mod?

Not discouraged yet.

Larry

Who said ultrasonic oscilation?

http://www.geofex.com/ampdbug/ampdebug.htm

[txstrat]

And should I even bother with the Orange Drop upgrade and the B-plus-lowering Zener mod?

From my own expierience I can say that ODs sound very sharp compared to such as Mallorys. If you're pleased with the sound you got, why not stick to it?
The voltages seem to be in reasonable span. I wouldn't change that either.

[MWJB]

"From a troubleshooting standpoint, I wish someone would post a schematic with ac signal levels from input to output. Starting with 10mV @1KHz at the input, show me what to expect as I work my way through the circuit. That would be very handy." - They have! Look at Fender.com/support/amplifier schematics.

This still won't necessarily help you tweak the tone if problems exist in layout/routing of wires etc. Post dc voltages for power & preamp tubes & post some photos of the chassis.

9 out of 10 players prefer a 12AX7 in V1, IMO...

[lmolter]

I had a talk with the service manager at my local Sam Ash store today. He used to work for Fender and he is very familiar with all of Fender's amplifiers. We talked for about 20 minutes and he was very excited to hear the amp. So I made an appointment to see him next Wednesday. I'm quite excited, too.

I'll post back here whatever we find is the matter.

Larry

[lmolter]

I had a very enlightening visit with the service manager at my local Sam Ash store today (remember, he used to work for Fender). He put the amp through its paces - low volume, mid volume, full volume, bright channel, normal channel, full presence, etc. The prognosis? He was awed. Sounded just like the original. He didn't even want to a/b switch it with a Bassman reissue. No need since it sounded so much better than the reissue. And you know, I never thought it had any problems, either. But since I wasn't a real good player, I need to be sure.

So now I'm going to tighten all the bolts and nuts, clean off the fingerprints, have a nameplate made, and sell the beast. Had two interested parties right in the store. And I got a request for a Tweed Deluxe, too. See everyone over in the 5e3 forum.

Thanks again for everyone's help and guidance.

Larry