6AQ5's in Express?

[Phil_S]

Wondering if someone has done it with 6AQ5. Probably the significant difference between 6AQ5 and 6V6 is that design center rating for max plate voltage is 275 vs 315. I understand spec is often ignored, but B+ at 395v is 120v over and that seems like a bit much. Is this worth trying with a lower voltage PT? I've been thinking it might work with 270-0-270 or 260-0-260. Or is there something about the high B+ that makes the Express what it is?

Also, if there are no intentions to ever use EL34 (incompatible socket), will it still be possible to get a decent result with a PT that has a lower mA rating? There is a rather large difference in the power requirements for the two tubes. I suppose we are looking at one pair of EL34 at maybe 35W and one pair of 6AQ5 at maybe 18-20W and this translates to a difference of perhaps 75-100mA?

[skyboltone]

400-410 VDC on the plates of 6V6GTs is done all the time. It's actually low by Fender standards. Running 6CA7/EL34s at that voltage borders on starving the plates. Everybody runs EL-34s higher. They'll take up to 1000 VDC.

Now, I've noodled the 6AQ5 question too. I think they would arc at those voltages. Somebody around here, I forget who, has built a guitar amp using 6AQ5s and loved it. I suspect you'll see a lot less headroom than with 6V6s though. Just a guess. If you build one with that tube, it won't be one right? Everything on the PS side will need to be redesigned.

[CaseyJones]

400-410 VDC on the plates...

I think they would arc at those voltages.

Been there, done that. I KNOW they arc at those voltages!

It may depend on what you use for sockets. Mine arced at the sockets, probably internally as well. The arc at the socket got my attention. Once a carbon trail forms at the socket the problem gets worse.

[Phil_S]

If you build one with that tube, it won't be one right? Everything on the PS side will need to be redesigned.

That is precisely the question. I can mess with it, but my theoretical knowledge isn't deep enough to qualify what I can do as design work. I was hoping for a hint or two. My gut tells me it isn't enough to lower the B+, but maybe it is. Maybe I'm over-thinking this? Oh, and thanks for confirming the expectation that I get fireworks at 400v with a 6AQ5.

The problem, as I see it, is that plate and screen supply need to go down to around 310 and 300 or a bit lower. This I can do with the right PT. The problem is that I have no idea what B+ supply for the PI should be and I have the feeling that I should keep it above the supply for the pre-amp stages which are in the 270-280 range and below the screen supply. So, do I run the PI at 285-290-ish and the preamps a little below that? I could manage to get the supply ladder running right, if I just had some idea of what is "right"! It all seems like a rather tight fit, stepping down about 5V at a time.

Then, I can't help but wonder if there is some mojo in the supply ladder R's at 18K2, 9k1, 9k1. The numbers seem so harmonious together. Guessing here, since I don't have a PT at this point, I'm likely to end up with 4k7's all across or something in that neighborhood.

Advice would be appreciated.

[philmanatee]

I built a six 6AQ5 amp, not a wreck though. I'm running about 350v on the plates and lowered the screens with individual 75v zeners, as per my friends suggestion. Before the zeners were added I was having a tough time finding six tubes that were reasonably close, I smoked a few! I used onetics iron, I believe his AC-30 power transformer, high def output, and a big choke. Phil

[skyboltone]

Ok PhilS:
Here's step number one. Download the PSU Designer II from this site, it's free:
http://www.duncanamps.com/software.html

I'm thinking 300 on the plates, so divide 300 by 1.35 or so and you come up with 222-0-222 or thereabouts. Get a 225-0-225 transformer and say 125-150MA for comfort. Your dropping string will be much different. The problem is always inserting a constant current load in the software so that you arrive at the voltages you want. Figure 1-2MA per 12AX7 stage and about 7-8MA for the PI. The pre-PI stage doesn't have much gain so maybe 1MA. Then insert the book values for idle current on the Plate and Screen supplies.

I've never fooled with fixed bias on a little tube like this so you may want to start out with cathode bias. Again book values. If you do go with fixed bias the book claims that a 6AQ5 is a 6V6 with lower plate potential so why not try the bias value that KF set for the express and see what happens. You must calculate though so that you don't exceed plate dissipation value which is a bit lower than the 6V6 IIRC.

Sounds like fun. Get some used tubes from Antique Electronics Supply before you plug in your NOS black plates though. This will be a time consuming and tweaking project. GOOD LUCK! Sounds like fun.

Dan

[Phil_S]

I built a six 6AQ5 amp, not a wreck though. I'm running about 350v on the plates and lowered the screens with individual 75v zeners, as per my friends suggestion. Before the zeners were added I was having a tough time finding six tubes that were reasonably close, I smoked a few! I used onetics iron, I believe his AC-30 power transformer, high def output, and a big choke. Phil

Interesting...I've got a 20H choke for an AC30. Maybe I don't have to be so picky about knocking down the B+ so low. I think I know where to get a 270-0-270 PT that I think will do a nice job. With SS recto, I'm too tired to calc, but I'd guess B+ for the plate in the existing design would be around 375. It wouldn't take much to knock it down another 25v or so. I imagine the choke would handle some of that.

BTW, I've got a nice mixed brand matched sextet. Used, not used up by any stretch of the imagination. At Va=~240, and Vg=-13, they dissipate about 10.5W and draw about 41.5mA. I can send particulars for the individual tests. If you are interested, let me know and I'll figure out how to send you some email.

[philmanatee]

I haven't used my express since I built the six banger, I gig quite a bit with it. It's solide state rectified, although I've got three 12X4's in there that I can switch to if needed. The only time I used the tube rectifier was a wedding gig where I had to drop my power so I pulled one rectifier tube and a pair of the 6AQ5's. It worked great, but the attack was a little spongier than I prefer. It's fixed bias also. Phil

[Phil_S]

Hey Dan: I use the TDSL all the time, but didn't know about the PSU designer. That's really a good tool to have.

I built a fixed bias VibroChamp so that I could test and match the 60 or so 6AQ5's that I've got. About half of them test like new or show reasonably strong results. Less than 25% tested on the low side, but all have life left in them and will give good performance. I'm good with building the bias circuit. I have it all worked out in the VibroChamp. BTW, it interferes with the Tremolo because of the cap value limit on the power tube in fixed bias. The tremolo on this amp is fabulous and I think I'll either be ignoring design parameters or changing to cathode bias now that my testing is done.

The higher value on Va will demand a more negative Vg than design center spec, which is Vg=-12.5, Va=250. BTW, except for PaMax, the 6AQ5 and 6V6 are like identical cousins. 6V6 PaMax=315, 6AQ5=275. That seems to be the "rub" here. I've got a PP 6AQ5 Gibson Scout where Va=305 or so. It is cathode biased. Nice sounding, too.

Thanks for the pointer on how many mA I need. I'm still trying to digest this circuit and figure why KF spec'd so many mA on the PT. I realize he is assuming the amp will use EL34's. I've been gun shy to cut it back, even though the math told me pretty much what you said. I had surmised that a 230-0-230 with 150mA would be plenty of tranny.

If I can work it all through, I just might do it!

[tele_player]

but I'd guess B+ for the plate in the existing design would be around 375. It wouldn't take much to knock it down another 25v or so. I imagine the choke would handle some of that.

The whole point of a choke is that it (ideally) acts like a short to DC, and resists changes in current according to its value and the frequency.

Obviously, a choke has some DC reistance from its wire - but just using a resistor is a more direct way of adjusting DC voltages.

[RJ Guitars]

Has anyone worked with some of the other 7 pin tubes? I've gotten a great tone out of a couple of them - the 6AK6 and also the EL95 tube. EL95's sound pretty similar to an EL84 to me... just less of it. I've gotten more of a 6V6 sound from the 6AK6's... again just less of it. 6AK6's put out slightly less than a 6AQ5.

I've worked a lot with creating very small combo amps for church and small venue gigs. These small tubes in push pull can still get pretty stinking loud... but I think they can sound better than the average SE amp.

The up side is that you can make everything light weight, they don't require a lot of iron and you can make a great little combo amp.

IMO - The down side is that they don't get as much interactive response like feedback until they start to get loud.

IMHO - My growing inclination is that "Loud" is part of the tone mojo...

I think they would make a great little wreck...

rj

[Phil_S]

My intention here is to see if a wreck can be built that satisfies without ear splitting volume. This is sort of like what the 18W Marshall is to the 50/100W models. I think you are right about "economizing" on the iron.

[doctord02]

Phil, I have a 6AQ5 wreck-like build in the works, but have not touched in in a while; it's laid out like a Liverpool, but has a few mod/tweeks taken from the Komets and various discussions here. It has the split anode fast/gradual switch, a pot for adjusting the 150k resistor to ground (before the 10k clipping stage), and has a master volume. I've also planned to put two of the power tubes on a switch for a half power setting.

I'm using a Ceriatone 18watt OT, and an Edcor PT (180-0-180 @250ma).

As I said, I havent touched it in a while, but will eventually finish her up.

[philmanatee]

My intention here is to see if a wreck can be built that satisfies without ear splitting volume. This is sort of like what the 18W Marshall is to the 50/100W models. I think you are right about "economizing" on the iron.

Definitely use fewer than six 6AQ5's. My six banger is about the same volume as my express. Phil

[Phil_S]

Your "six banger" is probably 50W or so. I was thinking a single pair is plenty. When I saw that Ken Fischer designed the amp for EL34 or 6V6, I immediately wanted to use a pair of 6QA5 so I could tame the volume a bit without changing the vibe too much. In every parameter that counts, the 6AQ5 is fully equivalent to a 6V6.

The open question for me is how best to lower the B+ plate and screen supplies without lowering the pre-amp and PI supplies. It is going to take a shoehorn to get this right. I guessing the ladder needs to supply 310-300-290-280-270 or something like that. The ideal power tranny would probably be around 250-0-250. Finding a reasonably priced one with enough mA is proving to be a challenge. I usually source the expensive parts from eBay or other used sources to keep the price down, so getting something new-in-box is a little out of my normal comfort zone. As it is, I know I am going to be buying a new output transformer. For that, I'm looking a Musical Power Supplies 22W 6K6 model.

In the end, I suspect that I will have to use a higher output PT than I'd like and use a big dropping R in front of the first stage. I worry though, that dropping volts also drops amps and if that's the case, then I haven't accomplished much except to heat up the chassis compartment. Lot's to contemplate here. If I could find some freeware modeling software that's easy to use, that would help a lot. I seem to flub the tube math on a regular basis and don't trust my own calculations. I must have some sort of mental block when it comes to that part of things. It must be my lack of theoretical knowledge.