power supply for a big bass amp-> sound ok?

[black_labb]

after realising how successful my last amp turned out, i think it might be time to make a big bass amp. i feel that i can make something that is worth more than the sum of its parts. im also looking to take advantage of what looks to be introductory prices on some nice looking torroidial transformers from AnTek inc.

im looking to put together an amp that runs 6 tubes at ~700v. im thinking JJ E34L's (more robust versions of el34's) as they seem to be well priced, but the amp should be able to run on kt88's and 6550's as well.

the power transformer i'm looking at is http://www.antekinc.com/AN-4T500.pdf . 500v with 2 6.3v windings with 5A each. because the 10A total will barely cover the power tubes (9A for e34l's, 9.9A for 6550's or kt88), i'd use a separate transformer to cover the preamp.
i cant seem to find any 6.3v transformers with a 230/240v primary winding, so im thinking that using a 6v transformer and rectifying it and using resistor to drop the voltage to 6.3v. this would also elimilate hum by using dc filiaments. im thinking using http://www.antekinc.com/AN-0206.pdf for that. 20VA should provide plenty of power for the preamp, which will probably be 3-4 tubes (3 12a_7's and one 6bl8 i'm thinking=1.35A).

instead of using a big dropping resistor for the plates, i will probably rectify the plate and the grid+preamp/pi separately, and run the grid/pre/pi rectifier into a choke input, which should lower the voltage to ~440v, which would be healthy enough for the grids.


does this sound like a reasonably robust design? am i running that power transformer too close to its ratings on the filament? i can live without using 6550's and kt88's (they are more expensive than the e34l's anyway), but i dont want to blow a PT. do torroidial trannies work well at close to their rated power? i know they have fewer losses due to the design, but it does seem quite close.

if im likely to smoke the PT i could always get a higher rated 6V tranny, and run one of the power tubes off of the DC filament supply, which wouldnt be a big issue. i would need to be wary if i swapped some 6550's or kt88's in that i would probably have to change the dropping resistor value to keep it at 6.3v.

any other concerns i should be wary of? i know i will probably have to double up some 4007's for the plate rectifier.

[black_labb]

another question, i saw when looking at the datasheet for E34L's that running a pair in class b, with 800v on the plates (400 on the grids) you could get 100w in class b. this seems normal, but what surprised me was that the p-p loading was rated at 11k? that seems very high. i would expect the load to be closer to 3.5-5k. is there a reason to have the p-p impedance so high? is this due to the high voltages? seems to change very drastically with the impedance at 4k at 500v, and 11k for 800v.

the pt was going to be a hammond 1650w (1900 p-p, rated at 280w). using 3 pairs i would have a p-p load of 5k7 ohms per pair. does this seem reasonable?

[Steve Conner]

Yes, the high p-p impedance is due to the high voltage. The amount of current any given tube type can supply is fixed, so in order to increase power, you must increase the voltage without increasing the current. That implies increasing the load impedance in proportion to the voltage.

If you try loading EL34s with 5k at 750V on the plates, you will probably send them to an early grave. Irrespective of the loading, the high plate voltage implies that you have to run a very low bias current, so the amp will be practically in Class-B, risking lots of crossover distortion and maybe a harsh and unpleasant tone. If you use tubes that can dissipate more, you can run in a hotter Class-AB mode.

I dare say 6V is close enough to 6.3. But if you buy a 6V toroidal transformer, you can add a few turns of heavy wire to get that extra 0.3V. Heck, if you're using a toroid already for your main power supply, you can just wind another 6.3V winding straight onto it. But I wouldn't worry about exceeding the 10A rating a little.

[black_labb]

adding the extra winding on the tranny is a good idea. i'll probably do that to get the extra 0.3V. i'm a bit hesitant doing a new winding around the tranny, as it could effect cooling.

not the best news of it not being able to get out of class b on 700v, but its good to know now. might use 6550's. just need to find them at a good price, as i dont really want to have to take them out of other amps.

[tubby]

i can recommend jjkt88s. they are reasonably priced (at least here in europe, can't speak for OZ) with around 27€
and they can take a beating regarding HV and plate dissipation.
used them in my recent project. tons of low end in comparison to an el34 or 6l6.

good luck with your project!

[Gregg]

EL34s don't sound to me like good bass amp candidates.
Concerning the power supply and stuff check out how it's done in most famous bass amps. No need to invent the wheel there.

[andrew_k]

May I ask how much you can get that power transformer for shipped to Aus?
Also regarding getting 6550s cheap, there have been some good deals on fleaBay recently for SED 6550C. I got a matched quad for 5 euro in December. Worth keeping an eye out.

[black_labb]

EL34s don't sound to me like good bass amp candidates.
Concerning the power supply and stuff check out how it's done in most famous bass amps. No need to invent the wheel there.

some big bass amps will have a separate filament transformer to the high voltage, unfortunately i cant find any filament supply transformers that suit 230/240V mains.
reguarding the voltage supples, some amps have a voltage doubler supply, where the 1/2 way point is used for the grids and the preamp (usually where there is higher voltages) and some use a dropping resistor.
i feel that a dropping resistor wouldnt be ideal, due to the voltages that would have to be dropped, and ohms law giving the amp some sag. i was planning to use a choke anyway, so using it in this way seems to work better. it saves using a big resistor adding heat, and should give a stiffer power supply. only issue i see is a separate FW rectifier is needed, but whats 2 4007's worth? doing it this way also means i dont need to worry about the full circuit rising to 700v before the tubes start to conduct, which would mean higher rated caps would be neccesary throughout the circuit.


reguarding el34's not really being suitable for bass, i can understand that. i personally like quite a bit of mids in the sound, and dont mind the more aggressive tone of el34's for bass (for me personally). i can understand that they are not for everyone though. apparently the jj e34l's have more headroom, so i guess they are the cleaner of them.

May I ask how much you can get that power transformer for shipped to Aus?
Also regarding getting 6550s cheap, there have been some good deals on fleaBay recently for SED 6550C. I got a matched quad for 5 euro in December. Worth keeping an eye out.

the shipping internationally is supposed to be 37 US$, but im considering shipping it to family in north america and trying to get someone to bring it over here next visit. i'm not in too big a hurry with this amp, as it may get started in november when i get more university holidays. i want to get the tranny at these prices though.

thanks for the tip on the sed 6550's, thats a stunning price!!

[black_labb]

i can recommend jjkt88s. they are reasonably priced (at least here in europe, can't speak for OZ) with around 27€
and they can take a beating regarding HV and plate dissipation.
used them in my recent project. tons of low end in comparison to an el34 or 6l6.

good luck with your project!

theres a supplier that seems to get jj's at a good price, but he doesnt show having some in stock. when the time comes i'll see if he has any.

[6267]

For a bass amp, I would consider a B+ regulator of some kind, google LM317 maida regulator for a common and useful one. This will prevent the B+ from sagging out when the amp is pumping watts at low frequencies. Also, the amp should probably be designed around the appropriate OPT, since you'll need the correct impedance & power handling. If you can get the primary Z down by paralleling enough tubes you may be able to use an off the shelf power toroidal.

[black_labb]

the regulator isnt a bad idea. i'll look into that. theres a hammond transformer that is rated for 280w, which will be perfect for 3 pairs of 6550's or similar. also has 4-8-16 ohm taps, which is pretty important for me.

[Steve Conner]

I'd say the regulator is a bad idea, since it adds unnecessary complexity and more things to go wrong. If you feel the need to regulate something, make it the screen voltage. Better still, lie down quietly until it goes away

Toroidal power transformers as OPTs are severely limited in power. A typical one, no matter how big it is, will have two 120V primaries, and you'll use these as your center-tapped primary. Therefore, each half of the primary will only take 120V RMS at 60Hz without saturating, and somewhat less at the 42Hz of a bass guitar's low E.

That means there's no point in using a B+ of more than about 250V, since any more will just saturate the OPT rather than making more bass. And it takes a lot of EL84s in parallel to make 400W. The best tubes to use with a transformer like this would probably be high-current, low-voltage types like 6C33s.

You may be able to overcome this by combining the transformers in series-parallel. Primaries in series, secondaries all in parallel to the speaker. If you think hard, you can come up with an arrangement that cancels the DC flux in each core and minimizes stray capacitance. But once you've bought three or four toroidal PTs, you might as well have spent the money on a real OPT.

[black_labb]

thanks for the help everyone. i'll probably go along with the original idea, but using 6550's or possibly kt88's

[andrew_k]

Has there been any progress on this? I'm currently building something similar, so I'm interested to read how you're going.

If you've not yet purchased an OT I'd suggest investigating the Hammond 1650R. Although rated at 100W it has been used successfully to put out 200W with a B+ of 750V - http://www.dogstar.dantimax.dk/tubestuf/dzart-4.htm

If you're after one in Aus send me a PM and I'll tell you how I got mine for a little cheaper

[GainFreak]

I'm also considering of building a 6xKT88 amp and was wondering how much the plates supply filtering should be. According to the schematics of different famous bass amps it goes from 50uF (2x100uF caps in series) in Ampeg SVT all the way to 1100uF (10x220uF in parallel and series) in Mesa Bass400?