Filter cap value question (in a Dynaco SCA-35 hi-f amp)i

[Wrongdog]

Okay there are other forums on the net to ask this but I figured some one hear could answer this question from a guitar amp design perspective:

What is the best values for filter caps in regards to tone and dynamics?? Why are guitar tube amps and old Hi-fi tube amps generally based on 40/20/20/20 type values and stages? I know Twins and big amps use 80uf etc...

A buddy just got this Dynaco SCA-35 intergrated hi-fi tube amp for free and wanted me to fix hum and put a grounded cord on it. The stock can caps were typical 60/40/20uf and a 50/50uf all at 450v. I was intrigued by Triode Electronics offering of unloaded cap board upgrade so I bought it. When it arrived with all the parts the 5 caps were ALL 200uf/500v. Is that a lot of capacitance? Don't you loose dynamics like pick attack if you over filter a guitar amp?? Why do bass amps tend to use larger values than a guitar amp generally?

Here is the link to the Cap board:

Dynaco SCA-35 Integrated Amplifier

I guess these are highly regarded entry level audiophile amps. I see there are board kits available to build them from scratch. Anyone have one or built one??

[Chuck H]

IME larger filters increase dynamics. Especially on lower frequencies. This isn't automatically good. WRT guitar amps, big dynamic bass capability can lead to wolf notes and woofy tone. The limitations of most guitar preamps, the speakers and even the instrument itself, and especially in combination, are prone to several conflicting resonant points in the bass and low mids. Smaller filters help squelch this effect. And though "dynamic" is often used as a positive description of guitar tone, truely dynamic amp response for guitar actually sounds cold, flat and uninteresting. Some kind of envalope of attack is needed to make the amp an instrument and not just the bland reproduction of what guitar and pickup are putting out. Contemporary overdriven guitar tone has A LOT more bottom end in the final, post overdrive EQ. The current trend seems to me more filtering in the power amp and more careful EQ sculpting in the audio circuit to get this result. Whereas once upon a time, when larger filters simply weren't available, guitar tone was molded by the limitations of the amp as a whole. Call it a happy accident. So , smaller filters being used in guitar amps probably continued (and continues) to be typical because it works as part of the tone/dynamic shaping formula.

JM2C

[Enzo]

The primary function of the filter cap is to smooth the rectified AC, so you have relatively smooth DC for the circuits.

The larger value the cap, the more it costs. So when Leo Fender made an amp, he was not going to spend extra money for a 100uf cap when a 20uf cap would do good enough.

Caps don't work in a vacuum, they are always in some context. Two 50uf caps in parallel is 100uf, two 200uf caps in series is 100uf. Now you could say look, one has four times the capacitance of the other, but in the context of your use, the two are the same - in both cases the amp sees 100uf.

In the old days, a design engineer would apologize for sag in an amplifier if you pointed it out, nowdays people are trying to add some in. The people designing the old stuff were not all about nuance, they were just doing whatever got the job done.

So what is the "best value" cap to use for tone and dynamics? Please define best tone and best dynamics. One guy likes beefsteak, another likes pork ribs. There is no right and wrong here. Then we start to ask what sort of current and voltage will be needed in the circuit. How sensitive to ripple is the circuit. Etc.

[Wrongdog]

Thanks Chuck. I kind of get it. So In this Hi-Fi situation, what do the big caps offer? I am Sure Ted at Triode and Sheldon at SDS know what they are doing, but I am curious about why and will be curious to hear the results when I get it finished.

Enzo - thanks. You posted before I finished this. So 200uf stages in this hi-fi amp is good?

[Chuck H]

As Enzo was good point out. The primary function of the filter capacitors is to smooth the DC that powers the amp. Bigger filters do this better than smaller capacitors. So, aside from any dynamic and tonal limiting advantage lower uf filters have in guitar amps, bigger is better. You DO want dynamics for a hi fi amp. As Enzo also pointed out, the value of the caps does not always tell you the value of the circuit. I didn't see a schematic for the replacement board so I can't say if you actually have 220uf capacitance or not. If you do, IME it's fine.

[Wrongdog]

okay - cool. I just wondered if it was over kill. The Triode listing did not give any details on the values they would send.

Dynaco SCA-35 Capacitor Board w/Parts

I noticed that the SDS site that the parts are listed now so maybe he updated it. These guys sure are into the Dynaco's!

[Wrongdog]

Tofu! yuk. In this case the best tone is to reproduce the album or recorded music accurately I guess. The circuit is extremely simple.

[Enzo]

For hifi, reproduction is the whole job, but not in a guitar amp - fundamentally different beasts.


And never lose sight of mojo selling - it is everywhere. You convince someone he needs to add immense caps to some amp and then offer to sell them, voila - make your own market for something. I can think of claims for "better" low end response by changing some cathode bypass cap. Never mind that the existing cap has a turnover frequency of 10Hz, but somehow moving it down to 5Hz will be "better." As if a guitar ever made 5Hz.

Where did the 200uf figure come from? As I read the product descrip[tion I don;t see mention of values. As I read it I would expect the parts supplied would have the same value as the original circuit multicap it replaces. They mention that one COULD increase the values - there is room for it.

[Chuck H]

If you go to the link and click on the description of the new board, then scroll down, there is a parts list. The value of the caps is 220uf. Nothing to say how it is wired. But as you say, I'm sure it's appropriate for the application.

[Wrongdog]

Chuck H is right - they are 220uf caps and they must be what SDS recommended so Ted at Triode supplied. Yes when I ordered it they were not specified and only after going SDS site did I see it.

I will stick it together and see! I guess lots of folks use these little amps and modify them. Some recommend bypassing the tone/EQ circuit?? From SDS site: "It's a nice sweet sounding unit that is very musical and enjoyable, particularly if the tone controls are taken out of the circuit."

Here is a how-to: Antique Radio Forums • View topic - Dynaco SCA-35 switchable tone control bypass

[Steve Conner]

In my tube hi-fi amp I have 100uF before the filter choke, and 300 after. When restoring it I got a good deal on some 100+100 dual caps that fitted the old can holes.

It has a diode rectifier. A tube one probably wouldn't be happy with that much capacitance.

In a hi-fi amp you want minimal hum and a nice reserve of energy, so more capacitance is better.

[kleuck]

What is the best values for filter caps in regards to tone and dynamics?? Why are guitar tube amps and old Hi-fi tube amps generally based on 40/20/20/20 type values and stages? I know Twins and big amps use 80uf etc...

Mostly because when amps here fitted with tube rectifiers, and each model of rectifier has a limitation when it comes to the first cap (50µf for an EZ81 for example) and obviously, the cost.
As you're talking bout a Hi-Fi amp, if your rectifier is a silicium diodes one, you can use bigger caps, and improve the bass.
A simple enough rule of thumbs is to use at least 1F for 1A, so if your power tubes are drawing 200mA at max power (see the datasheet) 200µf would be fine.

[Wrongdog]

Yes it is SS rectified. Board kit came with new 1N5408GICT diodes.

[Rhodesplyr]

I have a Dynaco SCA-35 and installed this cap board years ago. MAJOR improvement over the stock power supply. The larger capacitors create a stiffer supply and allow this little amp to produce dynamic transients that make it sound like a more powerful amp than it is.

When replacing the capacitors, I also strongly suggest new, ultrafast soft-recovery diodes, which have become common. The diodes Dynaco used in the 1960s tend to be very noisy by modern standards. Clean the selector switches with DeOxit D-100 and/or silver polish.

The SCA-35 can be tweaked to be a great-sounding amp with a few mods. Its output transformers are among Dynaco's best. It's tone-control implementation, however, is among Dynaco's worst, and the phono section can be noisy.

[Wrongdog]

Cool glad to hear this is worthwhile! Is there a favorite tone circuit bypass method you would advise?

I put the final install of this on hold due to a bunch of guitar repairs I need to do first but I look forward to results. The owner of the amp is our area go-to brass and reed horn repair guy and he is really appreciative of the hi-fi tube warmth on a budget. I may build one of these for myself if I like it too!! Would the Edcor trannies be a good modern route to go I wonder??