Overfiltering?

[bob p]

You can improve this dramatically by doing the rectification in SS diodes which have much lower incremental resistance, then running the resulting ripply DC through the tube rectifier into a second cap. The tube rectifier now carries the average DC current, not a series of peaks, so the average loss through the tube rectifier is much smaller than it would otherwise be.

thanks. that's essentially what i was thinking about when i asked this question:

@Steve and @Enzo, I have a question for you about this -- I designed a warmup circuit for the amp where the tube rectifier was taken out of the circuit and replaced by SS diodes for charging the empty caps. then, once the caps are charged and the tube rectifier is up to temp, the tube rectifier is added into the circuit. this effectively eliminated the "cold start" problem, but didn't do anything about the steady-state problem.

here's my question: how stressful would steady-state operation be on a warm tube rectifier that has SS rectifiers in series with it upstream, compared to a tube rectifier operating in steady-state all by itself?

i guess that i was on the right track. after reading your post, i guess i'd refine my question by asking this:

if you place UF SS diodes upstream, and the tube recto downstream, does it really make a difference what value of cap between is placed between the SS diodes and the tube rectifier? i understand that a cap interposed between them will provide significant smoothing of the rectified AC pulses, but bigger caps will result in less ripple voltage at the tube rectifier input, which should make for a "happier" tube rectifier.

from a modelling perspective, it appears that its always best to put the little caps before the choke and the big caps after, as this seems to result in less DC waveform ringing e when the supply has to deal with large dynamic increases in load.

thanks again for your help.

[Le Basseur]

Thank you Master Jedi!

Regards,
Le Basseur

[Bruce / Mission Amps]

Some of those big Altec-Lansing PA amps I've worked on, (used in some vintage Drive-In movie theaters) use two or four 572Bs for output power and four 5R4s, with parallel anodes, in a full wave bridge rectifier setup! HUGE current handling.

[bob p]

something like a 1570B?

[lowell]

bruce, i like the ideas that you mentioned in another post about creating the brute force input filter and taking the other sections of the rail off of that radially. my preference for the 5E3 is to create an additional node in parallel to the last one on the fender schematic, and to use that node to supply the first gain stage (instead of supplying two gain stages off of the last node). in this approach works great to eliminate motorboating at high control settings when you run the amp with higher gain tubes like the 12ax7.

getting back to the idea of Pi filter design, i'd like to add a couple of comments from a hifi perspective. its really important to size your chokes appropriately for the current demands of your circuit, both in terms of inductance and rated current. depending upon whether the user is designing an amp that will have small or large current drain on the PSU, you might want to have a 5H choke that's rated at higher or lower current.

one of the things that can be confusing about chokes is that their inductance varies greatly with current, so spec'ing a choke can be difficult. optimally, you'd want a choke that has lots of inductance at lower currents (to store energy), that drops to a minimal value at the circuit's maximum current. this helps to prevent too big a choke from imparting "ringing" in the DC voltage rail in response to transients. if you've got too big a choke (in Henries), or a choke that is rated at too high a current for your circuit, it can impart really UGLY ringing artifacts on the DC rail. (Duncan's PSUD can demonstrate this well.)

picking out the right value of a swinging choke has always been conceptually difficult (at least for me) as nobody's ever developed a good mathematical model that completely explains all aspects of the problem. for this reason, choke sizing has always been a more of an art than a science, or something that tends to work out better from experience than from math. (stokes, note this occasion where i go on the record saying that math is no good! )

one reference that i've found particularly helpful is a nomogram for choke sizing that i found in one of Norman Crowhurst's books on hifi design. using the nomogram, you specifiy how much current your circuit draws at idle, and at what voltage, and you specify how much current your circuit draws under full load, and the voltage. the nomogram will tell you the optimum specifications for a swinging choke, both in Henries and the appropriate current rating. best of all, no math! (if anyone's interested in the reference, lemme know and i'll post a link).

Yes please post a link! This sounds useful.

[daz]

Wow, what a great thread ! I'm glad the last posted found this and brought it to the top or i'd never seen it. I have recently been playing with my amp's PSU a lot and really came to see just how important it is to get your tone. the mention of parallel nodes interests me because mark sampson used this in the chieftain, and amp who's PA i used as a model to build mine. Not exactly, but close. Those amps of course have a rep of killer tone, and not having noticed PSU's like that before i wondered whether i should try it. I ended up not, but now i'm wondering whether i should try.

[tedmich]

I always assumed the amount of filtering in guitar amps was a complete cost issue, especially with SS rectifiers. The PV JSX has almost 600uf of filtering (340uf, 110uF, 100uf, 22uf, 22uf) but the really big numbers are on high fidelity tube amps. The VTL MB 175 Signature monoblock power amplifier uses 2 x 3800uf to filter its supply feeding its 175w push/pull tetrodes, while the Cary CAD-1610-SE monoblock power amplifier, with 12 x 1200µF/450V (!) of filtering on its power supply for $20k at 45w is an obvious extreme.

So do parallel HV rails with big filter caps give a guitar amp that doesn't sound lifeless and sterile? How much is too much filtering?

Maybe try these?

Arcotronics polypropylene capacitor. 68 µF, 800 vdc, $35, 3"x3" 100k hr @ 720VDC, 8mm studs!

To toss out another HiFi vs Guitar tube conundrum, Billy Zoom of X (also an EE I believe) states that OT UL taps just give better pick dynamics and are essential additions to the custom amp he built and has used for decades...opinions?

[lowell]

nice looking cap but maybe overkill and too pricey for a tube amp?

I think if this thread came to a conclusion, it'd be that filtration is largely a matter of preference.

That peavey amp looks as though it needs more filtration cause of all the gain stages. Also Satriani's style calls for tight lows.

[bob p]

wow. i re-discovered this thread after being away for a couple years while i've been distracted by other things. i seem to have forgotten a lot of what i used to know. i hate it when that happens.

Yes please post a link! This sounds useful.

Here is the goldmine: Pete Millett's DIY site. lots of good scans of hard to find, classic tube texts. (somewhere in there you'll even find some McIntosh schematics that I corrected and cleaned up with the gimp.)

The book that I was referring to was one by Norman Crowhurst. I think it was High Fidelity Circuit Design but I can't say for sure.

Here are some direct links:

Crowhurst - High Fidelity Circuit Design 1

Crowhurst - High Fidelity Circuit Design 2

If they don't work for you, try this:

Pete Millett's DIY Audio pages

HTH

[GainFreak]

I hope this doesn't sound wrong, but my mental picture of how filtering works in guitar amps is that, with the regular values of caps used in old amps like the tweed Deluxe, when you crank the amp you get sag that sounds like a compressor. (Technically it IS a compressor/limiter, as the sagging screen and plate supplies modulate the gain of the power tubes as well as the maximum output voltage.)

This is all good but what if we talk not the usual Fender-ish stuff but high gain amps instead and no tube recitifiers but only SS brigde rectifiers?
Certainly one of the things I wouldn like in addition to an already compressed highgain preamp is more compression but a good attack and transient responce from the power amp.

[bob p]

its always been my personal opinion that the best thrashy sound comes from amps with nice stiff supply rails... more along the lines of a HiFi PSU than a blues guitar amp.

of course if you believe that then the next logical step is to say that a triple recto configuration is just pointless marketing and that it provides no real benefit other than to provide ambiance lighting for the guys on the backline.

[defaced]

Agreed. Oversized power tranny so you don't sag the transformer and oversized filter caps so you don't sag them along with diode rectification makes a solid power supply. You may have issues with inrush current with this setup (I and black_labb have both had problems with this with similar setups) but that's easy enough to fix, there's a thread on it with solutions provided.

[GainFreak]

Oversized power tranny so you don't sag the transformer and oversized filter caps so you don't sag them along with diode rectification makes a solid power supply.

I read somewhere a Hi-Fi stuff where PT are wound with much thicker wire thus considerably decreasing the winding resistance so it won't sag.

You may have issues with inrush current with this setup (I and black_labb have both had problems with this with similar setups) but that's easy enough to fix, there's a thread on it with solutions provided.

Did you mean something like this:

RHOPOINT|SG160|THERMISTOR, NTC, SURGE-GARD | Farnell United Kingdom

If not can you point to the thread in question.