New build advice

[Phil_S]

Not sure where to put this or exactly what advice I'm seeking, but here goes.

That 375-0-375 100mA that someone gave me and that puts out 855VAC unloaded is burning a hole in my workbench. Thanks for comments on the earlier thread about using a choke. I've got a suitable choke now. With choke input, I think it will tame the beast to something just a hair north of 300 VDC. I expect it will produce 150mA with choke input, so I'm in safe territory for EL84's and certainly for 6V6's or 6AQ5's. So, I'm bent on using it.

From a tonal perspective, what might I expect using a choke input? Anything I'm likely to notice? Squishy? Tight? Bassy? Remember, this should have available enough mA that the circuit won't be demanding more than what's available (re: sag.)

I'd like to build something Fender-ish. I'm looking for something that has good clean tone, with good low end and high end. I'm OK if I get some dirt when it's pegged. Is there a fit? Suggestions?

How OK is it to take one of the 6L6 Fender designs and "downgrade" it for one of the lower power tubes. Does this have a major impact on the amp's tone? I'm wanting to mix and match pre and power sections, but I don't really understand this from a tonal perspective. I notice, for example, Fender deployed the same designations across a number of it's amps, for example AA763 coule be a BF Bandmaster, Concert, BF Deluxe with or w/o reverb, Pro, Showman, Super Reverb BF, Tremolux BF, Vibrolux BF, or Vibroverb BF. All of these are 6L6 amps EXCEPT the BF Deluxe. I recognize the EL84 is a much more significant departure from 6L6 than 6V6. What gives if I do that?

I've given some thought to rectifier. I am intending to build it with a ss rectifier. If B+ is too high, I'll leave room in the chassis layout to drill and retrofit a tube rectifier. The PT has 5V @ 3A available. I expect it will run high -- close to 5.5V -- but not too high if I want to use it. I'd probably want to go for a 5U4GB as a measure against allowing the filament voltage to go higher in addition to the expected B+ voltage drop.

I"m not stuck on a particular brand and could just as easily move in another direction. I've got a couple of Marshall type amps, so that's enough of that. I'd be open to almost any 2 bottle amp or cutting a 4 bottle down to 2.

Thanks for you whatever you might be willing to offer in the way of advice or suggestions.

Phil

[JazzGuitarGimp]

From what I understand (which is still very little), the output power (and hence, power required from the PT) from an amp is more dependent on the B+ voltage and the OT primary impedance than it is on output tube type. If you plug a pair of 6L6GC's into a Deluxe Reverb, you're still going to get about 22 watts out. You might consider a duet of 5881's into a 5K OT for an output power of about 25 watts (with a B+ of 300-ish volts).

[lord preset]

Re the choke's effect on amp characteristics this seems to indicate that it would tend to make the amp sag less.

http://www.tonesecret.com/?p=1622

Since everything I find on the Internet is true, we can say that the choke input in combination with SS rectification will yield a very tight, even uptight amp.

Re. preamp designs vs power tubes, there is no reason not to do whatever you want. An AB763 preamp is going to sound different with 6V6GTs vs 6L6GCs - but whether the difference is significant or whether that's good or bad is subjective. The difference between 6L6s and EL84s will probably be even more noticeable. That said, I suspect that much of what we think we know about power tube tone is a function of the preamps we tend to hear them with.

[Phil_S]

Good, helpful stuff! I'm not necessarily looking at AB763, just citing the example they did one of the group with 6V6's.

I hadn't thought of a pair of 5881's. It seems I acquired a Dukane power amp a while back that was ANOS, from a retired repair man. It came with the original ANOS 5881's and a nice 12AT7! I could also use those crappy Russian 6L6 wanna be 6P3S that I've got a pile of, or maybe I can troll through what I have and find some 6L6G or GB.

Good read on chokes! It probably has a few half-truths, but it is easy to agree it stiffens up the power supply. I am starting to lean towards the 5U4, though that one is so robust for what I'm going to do, it won't much matter.

Thanks!

[rp]

I am starting to lean towards the 5U4, though that one is so robust for what I'm going to do, it won't much matter.

I can't imagine what you can do here w/ a ss rectifier and only 100mA. Definitely 5U4 if you really have the 3 amps. There's more than one 5U4 ie G, GA, GB not sure if GC, one drops like 40V and one 50V. Instead of a choke btwn C1 & C2 you could use something like a 1K/25W resistor like an Express or some Traynors.

So there you go, might not even need a choke input if you build a high plate voltage 6V6 JTM-50 type, I don't recall the year but the early '70s bright grinding hot ones. Might be a cool experiment - a bitching hot Marshall but with 6V6s.

NOS 6V6s can handle 470V (at least Fender thought so) and supposedly one of the modern JJs versions can. If it's for you who cares about shortened tube life. So, if you can get it around 450-470V w/o a choke input you should be able to adapt a circuit for ~100mA.

I can't help with exact tubes, but maybe there are some esoteric television tubes that can handle high plate voltages and are rated for only 20W/pair. Might ask over on DIYAudio forum.

[Phil_S]

Interesting thoughts, RP but I think you pegging B+ too low. I'm guessing this beast of a PT will do as follows for B+ with the 5U4GB, of which I've got several used USA tubes that should still work well:

Unloaded 855VAC; guess 95% of that loaded is 812VAC. Half that times 1.3 with a 5U4GB gives 528VDC. IMO, this is not something that will be easy to use at 100mA. Plate voltage will be too high even using 6SN7GT, which are rated for 450V, and I've already done one of those very near the upper limit (nice amp, too.) Even if I got a 10% drop in a loaded circuit (guessing this is the outer limit,) the B+ would still be 500VDC.

I am open to any solution that would work. I drifted towards the choke input because it offers the best solution for dropping B+ AND it has the added benefit of raising the mA! Depending on whose formula you think is the right one, B+ should be in the 300's. Choke calcs:

A) According to Hammond's well known guide, choke input yields .45x VAC at 1.54x mA. That's .45 x 812 = 365 @ 154mA. Swapping the ss rectifier for the 5U4GB cuts about 11% off the B+ = ~325. That's really where I'm thinking this goes from a practical perspective.

B) General rule of thumb for choke input is that B+ is .707x. If I understand this one correctly: 812/2 x 1.414 for ss recto = 568; 568 x .707 for choke input = 402. Deduct 11% for the tube recto = ~358VDC.

C) Working the tube rectifier with the voltage drop factor instead of the 1.3x multiplier (1.3x vs. 1.41x is the source of my 11% differential): 406 x 1.4 = 568; deduct 50V per the tube spec = 518. Knock that down by .707 for choke input = 366VDC.

These numbers are not far apart; take your choice 325, 358, or 366. Lots of figuring. Did I do this right?

I have a feeling I should build out the power supply and see exactly what it does. It may be a while until I get to that. Doing it would allow me to swap the 4 chokes that I have to see the differences.

[rp]

If it must be a choke input then how about the pre-eminent choke input guitar amp - the tweed Bandmaster but with 6V6s? The 5E7 was good enough for Who's Next and 1st Montrose, supposedly, so it's not just for blues. Maybe use a 6.6K BF Deluxe OT with 4/8/16 taps to play with and try and mimic the 6K/4ohm seeing 2.67 ohm of the orig?

[pdf64]

The PTs secondary regulation may be slacker than 5%; PTs intended for tube rectifiers would normally have a sufficient degree of their required equivalent series resistance incorporated into the HT winding.
So it may sag rather more, perhaps at least 10%, maybe even 15% at full load.

[rp]

Not sure the consequence of pdf64's point on the amp's playability but I checked choke prices and the bigger Hammond chokes are about $50. Maybe just give that PT an adiós on ebay, get hopefully $30 plus the $50 towards a PT you really want? Might be a hard eBay sell though, people buying PTs know what they can use, they'd wonder what to do with it too.

[rp]

ooh! Big single ended - KT88? But saggy Bandmaster sounds more fun.

[Phil_S]

I have the choke I need. I've actually got more than one and all were acquired super cheap, some for $5 and some for maybe $20. I'm an iron junkie. I have a problem and need help!
Thordarson T5319 8H 120mA
UTC 81854 12H 125mA
Mil/CET W 1631 8H 150mA
Thordarson T-19C42 12H 200mA
Stancor C-1703 4H 250mA
Drake for Vox AC30 20H 100mA
So, I might actually be buying a few of those "loser" PT's that no one wants.

PDF64: thanks for your insight on slack regulation. I did not believe this PT would sag that much. Your comment prompted me to do a little more math. Line voltage here is probably 6-8% above what is was when they made that PT (115 vs 122-125.) Assuming that discount factor, unloaded at 7% less changes from 855 to 795. It's only 6% down to the rated 375-0-375.

The 5E7 Bandmaster looks like a very promising candidate. I might even consider using those ANOS 5881's. I don't know what I'm saving them for. Do I have to cover it in tweed?

[didit]

8H 150mA seems right. Note that 5E7 is PI input with 16uF before & after the choke. Either breadboard build & test power before commiting or model in design software -- eg., Duncan's PSU app. Doing both will give best feel. Prototype needs solid dummy load for accurate results.

Best .. Ian

[pdf64]

The dc resistance of the choke may also affect the HT regulation, eg that Vox one may be ~500 ohms, which could be significant.

[rp]

BTW Phil if you're not familiar the 5EXA series tweeds the BM, Super, Pro are identical, first two used the same OT for sure, I'm not sure about the Pro, only difference w/ the pro is the NFB resistor so maybe it had a different 8 ohm OT? But keep in mind the mojo here was the odd 6k underspec'd OT and the BMs mismatch for double extra mojo. Just want to note that a more typical husky 2X6L6 Fender OT might mess w/ the magic. BF Vibrolux OT should be appropriate too if you want 4K rather than 6K.

I'm also thinking if you build it and the PS turns out to be trouble you can always lay out for a proper PT and you still have a Tweed Super/BM/Pro - not the end of the world. Though that 850VAC/100mA PT will still be keeping you up nights.

[Phil_S]

PDF64, you are spot on with the Vox. DCR readings (1st) and weight (below):
290Ω Thordarson T5319 8H 120mA
1#3oz/540g

253Ω UTC 81854 12H 125mA
2#3oz/1kg

179Ω Mil/CET W 1631 8H 150mA
2#9oz/1.2kg

124Ω Thordarson T-19C42 12H 200mA
4#13oz/2.2kg

60Ω Stancor C-1703 4H 250mA
3#15oz/1.8kg

445Ω Drake for Vox AC30 20H 100mA
2#3oz/540g

Obviously, choosing a choke is a new experience for me. Are you saying I want low DCR or high DCR? I suppose high DCR means more turns and/or heavier wire gauge. Both the T-19C42 and the C-1703 are significantly larger that the others, the T by the height/width, the C by the depth of the stack.