6AQ5 Express -- I've got a problem

[Phil_S]

Greetings: I've completed the build of my 6AQ5 Express. Many things seem to have worked out just fine, even though I'm off the reservation with layout and some components.

The problem I seem to be having is that none of the pot controls work. The amp is passing signal at a modest "bedroom" volume, but the volume control does nothing. I metered the pot and get 470K on each side of the wiper, so I'm assuming the pot is OK. I've been over the wiring and know that I've munged this somewhere. As you probably know, checking your own work is typically problematic.

Any thoughts would be appreciated.

Please ignore the missed plate wire for V1 and the missed B+ jumper on the 11th & 12th terminals on the top of the board. These have been corrected since taking the pictures.

Here is the voltage chart and a couple of pictures:

[Richie]

looks like your missing a wire connection off of v1..pin 1 back to the board

woops should have read the whole post.. sorry..

usually if your pots are not making any changes, the ground is not connected if there is a ground connection on the pot....maybe a loose solder joint etc.. also the ground off the middle pot, make sure that is good

[Phil_S]

<d'oh - headslap>
Richie: Sometimes you just need someone else to kick you in the butt. No cathode grounds on V1. Thanks so much.
Phil

[Phil_S]

OK, done. Now it works right! Time for a few questions about voicing.

I'm using a Korean PRS with soapbar P90's through a Celestion G10L-35. This amp is frigging LOUD. It also sounds very harsh to my ears. I don't think I opened up the guitar or the amp volume past about 1 or 2 and I put the treble and mid pots to 1 or 2 and nearly maxed the bass pot. Presence squeals below 4 or 5. (You all realize there are no actual numbers on the amp.) It is impossibly bright with the bright cap engaged.

It this harshness likely tubes or coupling caps? I'm pretty sure I made some bad choices on caps as I ordered from Mouser, If you've ever ordered from Mouser, you know what a PITA their website is, and I received a few "mistakes" like those WIMA 470pf caps meant for PC mount.

Resistors are KOA Speer -- yes, I read the thread on those, but after I ordered.

Present tubes are V1 GE 7025, V2 Siemens 12AX7, V3 EI 12AX7, V4+V5 Sylvania probable ANOS.

To be clear, this is an amp I think I could really like if I could tackle the bright thing.

Suggestions would be appreciated.

[skyboltone]

Hey Phil;
First thing, the Celestion 10L is not my idea of the ideal low end speaker but aside from that.....

Forget about playing nice and softy for a minute. Turn the guitar knob down to 0 and then everything on the dashboard at 1 o'clock with the presence dimed.

Now start turning up the guitar knob until you can't stand it (I use ear plugs). See if the base starts to fill in at this point. These are loud amps, no way around it. I used RN-65D resistors everywhere and 6PSS spragues everywhere except the treble, brite and PI snubber. Those are Mallory silver mica.

Increase the size of the PI snubber.
Use shielded cable on the volume to V1B run. Very important.
Increase the size of the slope resistor a tad. This will lower the knee of all three controls. I went back to stock but it's interesting what happens. You'll also be increasing your insertion loss into the stack so things will get a bit more managable volume wise.

Now this is purely low level technician NOT ENGINEER stuff but I think some of the harshness of these amps is due to the unbypassed cathode resistor on V2. It's also the source of much of the noise. That much is straight out of the book. I had enormous luck using a filament ground balance pot (250ohm) between the filament wires. It lowered the noise floor about 100db (I know, I know, alot! then) KF had the luxury of playing with gobs of tubes to find the ones that matched K to F capacitance and sort until he found the right combo. Try the balance pot. Be sure to lift the center tap on the 6.3V winding. I think you'll like what happens. Will the tone change. Only in so much as those harmonics of 120hz will be affected.

<edit> I'm not too overjoyed about flying your output tube grid wires either. KF layed his right on the deck. I do that and twist them too. YMMV <edit>

Beautiful wire job man!
Dan H

[Phil_S]

Hi Dan,
Thanks for the comments. I don't know if you can see the artificial filament CT, but its in the upper right-hand corner, directly to the chassis. There is no filament CT. The PT and OT come from www.musicalpowersupplies.com and they are basically meant for an 18W build or something similar. This amp is not particularly noisy and for a high gain amp, I think it's darn good. Really, I should take the Guild SF4 out of the case and give it a whirl.

AFA caps, I had difficulty sourcing all in one place. As you know, you get killed on postage otherwise. I'll see what I can do about changing some of the caps.

Is there something unholy about adding a small bypass cap to V2? After all, I'm building for me. As you can see, I'm not all that hung up on authenticity. BTW, shoehorning in the power supply was challenging. This is a 17" x 6" chassis. I wonder, what's the size of a "regular" one?

This build begs for a comment about the grid wires. With the 1K5 sandblocks and other stuff at the base of a 7-pin socket, it looked to me like I was out of room, so I used the air space. I know this is iffy. I can look at twisted grid wires under the board, but they will still end up at the top of the grid stoppers. Actually, that is probably one of the easier things to change. Maybe with a bit of heatshrink, I can get it to all lay down. I'll see.

I'm afraid I'm just an amateur. I avoided the use of shielded cable, figuring it is just for noise supression and I wanted to see what the amp does. I realize, however, shielded adds a bit of capacitance, and that might desirable. I might swap the grid runs on both halves of V1 for shielded.

I can see this is going to be a tinkerer's special for a while. I'm glad you liked the wiring job. It could have been better. I suffer from not seeing around the corner, meaning I don't always assemble in the most efficient order. It can make some of the wiring very challenging, resulting in things like flying grid wires.

Overall, this was a satisfying build.

Regards,
Phil

[skyboltone]

Hi Dan,
Thanks for the comments. I don't know if you can see the artificial filament CT, but its in the upper right-hand corner, directly to the chassis. There is no filament CT. <snip>


Overall, this was a satisfying build.

Regards,
Phil

That's just the point. With the artificial center tap you end up with both filament legs 100ohms above ground. Yes, I know. It's commonly done that way. But this is different.

With my build the 250 ohm pot is 3/4 or more toward one side and most of the noise and harshness disapeared. Hard to sell this thing. Ken didn't do it. HAD doesn't do it. But fender did it. It's in the tube handbooks, the Radiotron Designers Handbook, old ARRL handbooks etc. I had never done it before but I'm converted. It works.

Dan

[harryk]

Hi Phil! After finalizing one Express with 6V6 tubes and one Liverpool, I am now aiming to do a configuration with Express preamp and a push-pull powerstage with 2xEL95. This should give some 7 W output which is loud enough for many purposes. I am very interested to see if I could use your power amp schematic as a starting point to my PA with those EL95 tubes. Could you be kind and share your schem with us? Regards Harry

[Phil_S]

Harry,
The iron comes from www.musicalpowersupplies.com
He sells a 275-0-275 and a 6600K 4/8 ohm. These are what I used. (I'm not sure if 6600K is an appropriate plate load for EL95, as I haven't checked the spec sheet. ) He says he's going to put a 16 ohm tap in the OT. Schematics of the trannies are on his sight. The PT has +/-5% primary taps and I selected the two outer legs to reduce output at the secondary. I ended up around 262-0-262, but, as you know, this changes daily with line voltage.

I made only one other change to Francesca's power supply. Instead of the 18k2 between B+2 and B+3, I subbed a 9k1. All three of those 9k1 resistors in the power supply ladder are 3W metal film.

Although calculations suggested I would end up with around the right voltage at B+3 for the preamp, I was shocked that it came up at 297V, which is really spot-on. What I can't figure is why B+5 is a bit on the high side. I have not checked it since I fixed the cathode ground and suspect it has now dropped to something close to the Kelly 90 schematic of Francesca, which you can download here at Amp Garage.

I'm really off the reservation with layout, but otherwise I pretty much built Francesca, at least in terms of the component values. The concept was knock down the power supply, keep the pre-amp. It means you need to operate the power tubes at something above 300v. I think that will be a real challenge for EL95. It is over the design rating and I suspect, as with the 6AQ5, that Va=330 is pushing the envelope. Good luck.

Phil

[harryk]

Phil! My PT iron is 250V/110mA which hopefully gives loaded some 300V to EL95 plates. That is maximum according data sheets of that tube. I dont mind if my pre amp volumes are less than in original Express because I do not seek the ultimate clean sound out of it and lower voltages mean often more compression to pre amp which is good with distorted sounds. What resistors values do you have in your screens in power tubes and do you have any master volume in it? I am aiming to have a PPIMV to my amp. I have used it in many maps with success despite many builders do not like it.

[Phil_S]

Harry,
I think you will change the character of the amp considerably if you allow the preamp voltages to drop too much. The good news is that I think your 250-0-250 built out according to the schematic will give you in the neighborhood of 310-320. Remember, you will not be running the power tubes at more wattage than is recommended, so I do not think you need to be concerned about going over 300. I do think at some point those small bottles just can't take the heat, but not at 320.

Also, 110mA is plenty of juice for 2 EL95 + 3 12AX7. All together, this is maybe 52mA on the HT secondary.

The trick, IMO, is to drop just enough to get B+3 in the range of 290-295. To do that, you can make a pile of fancy calculations, but my gut tells me to use a dropping resistor in the supply ladder of something between 5K and 7k5 between B+2 and B+3.

As I said before, the amp is a faithful copy of the Francesca schematic found here at Amp Garage. https://tubeamparchive.com/viewtopic.php?t=1408
The main departures are layout and the type or brand of component, but the values are the ones on the schematic.

Screens and grid stoppers are stock, 1K 5W sandblocks for the screens and 1K5 2W metal film for the grid stoppers. No master volume. I have low wattage amps for low volume, so MV doesn't interest me.

Regards,
Phil

[harryk]

Phil! Why I am aiming to use that PT is the reason that I already have in my stash. I could buy also another one , let`s say 2x260V/70-90mA but I think that I manage with this present one. I have built tens of tube amps from Fenders to M-boogie clones and my own variations. If you want to see the newest ones, please check my web pages www.bluetone.fi. I try to capture Fransesca sound like you but you never know what you get before you start to tweak and modify them. I have already built one Express/Fransesca and one Liverpool with good success. Thank you of sharing this information with me. I think that these low wattage tube amps are coming to "Fashion" more and more these days. I have played 30 years in many rock bands, still playing once a week and my hearing cannot stand anymore any 100W tube monsters. Regards Harry

[Phil_S]

:Why I am aiming to use that PT is the reason that I already have in my stash.

I think that is the absolute best reason to use it. I have built several amps from junk, including a Champ, 5E3, and a JTM45 and a TMB18W. Some of these have more new parts than others. All have recycled iron and the only new chassis in the group is the 18W.

This TWX is actually the first one I've tried from all new parts in quite a while.

Did I misunderstand? Is this 125-0-125 @ 110mA through a doubler for 250-0-250 @ 55mA? It still works. Just curious.

Let us know how it turns out.

[harryk]

Phil! My PT is 250-0-250V/110 mA. Maybe I wrote it wrong because it is Friday and pretty dark here in Finland. I suppose that PT should be ok for my "Express Junior" project. I am just now finalizing my modified 5E3 combo project with 2x10" Weber alnicos. When I get it done, I will start this project. Regards Harry Bluetone

[Phil_S]

Harry:
Yes, I imagine it's dark there this time of year. Even where I live, the sun is going down at 4:20pm (16:20). I suppose you only get a few hours of daylight. Under those circumstances, you should be excused from lots of stuff. I don't know how you get through it. It sounds like you have a nice PT to work with.
Phil