vta 70 can capacitor

Hi
There are two can capacitors available from dynakitparts
Both are 80,40,30,20, one is 525 volts and the other is 550-600 volts.
Which one is beter for the vta 70?

regards
kost

Kevin at Dynakit is great at helping with questions, he would be happy to guide you I am certain.

I think the general consensus I have seen here is that the Dynaco tends to have a large rush up in volts at turn on, so likely most would say that having the higher rated cap would be a nice extra measure of protection. The capacitance of the cap is still the same either way, the higher rated one just gives more protection against current rush when the amp is initially turned on.

wolverine

I'm going to order a 550vdc cap for my st-70. However, I've been warned going above 50uf on the first section can put my 5AR4 at risk. What is the benefit of the 80/40/30/20 @ 550vdc versus the 30/20/20/20 @550vdc ? Does the 80uf in the first section really pose a risk to the 5ar4?

Thanks!

janka wrote:I'm going to order a 550vdc cap for my st-70. However, I've been warned going above 50uf on the first section can put my 5AR4 at risk. What is the benefit of the 80/40/30/20 @ 550vdc versus the 30/20/20/20 @550vdc ? Does the 80uf in the first section really pose a risk to the 5ar4?

Thanks!

Good question concerning connecting the 80 uf as first section. There are really two good reasons for keeping the first cap section relatively small, say 30uf and not much more. First it is a matter of rectifier tube reliability. Second it is a matter of performance. But there is a great performance advantage in putting a 80uf, or even larger, cap in the rear lug position. I'll start with the first idea.

The first section capacity reactance and the resistance of the power transformer high voltage windings are what primarily determine (and limit) the rectifier peak inrush current during an inrush event. Ordinarily, the relatively slow rectifier filament warm up prevents inrush at turn on, but if the amp gets hot switched or if your utility company should briefly interrupt your power, you will get the inrush event, guaranteed. During a brief power interruption, lets say 1 second or so, the first section cap voltage will fall to something less than 100 volts but the filaments of the GZ34 (5AR4) will still be plenty hot. When the brief power interruption is restored the inrush currents involved in trying to peak up the first section from a voltage less than 100 volts vs the usual 400 volts that ordinarily would have been there will easily exceed the 3.76 amp limit set by GE (for example)and you will be at risk of finding the coating of the rectifier cathode spread about the interior of your now useless GZ34. The color of that now powderized coating is white, BTW. Now for the performance advantage of keeping the first section in the 30 to 40 uf range.

There is an optimal value range for the first section if the highest possible operating B+ is to be had. Oversizing the first section will lengthen the time constant of the first section, composed of secondary winding resistance and the capacitance of the first section, and result in a lower peak voltage on the first section and a consequent reduction in B+ value. I have tried dubbing in various capacitance values in the first section and found 30 to 40 uf yields the best results.

There is, however, a really great advantage to upgrading the rear lug, or "C" section capacitance. The original 20 uf cap in this position allows some very substantial voltage swings in the plate B+ for the power tubes. I have measured voltage swings as high as 40 volts on this point when the amp is just driven into AB1 mode. To say this causes compression would be an understatement. Even at low listening levels I can hear noticeable improvement when this section is upgraded. 80 uf will reduce music induced power tube B+ voltage swings, all other things being equal, by about 75%. Personally, I prefer and use a "helper" cap of 210 uf.

IMHO, upgrading the rear lug capacitance is the most bang for the buck as upgrades go and you are sure to be pleased with the sound of your "new" ST-70. Hope this helps the ST-70 community. Rock on!

Hi Dynacojoe:
Thank you for that lucid explanation of what is going on electrically within the quad cap section during power outages and fast on-off cycling. This is some of the most useful info to me I have read in the forum lately.
I didn't realize how important control over this part of the circuit is and how it could affect the sound quality. We all have a variation of the quad cap in our dynaco circuitry and it is therefore relevant to us all. Could you provide a small diagram or schematic of the supplemental cap you use for clarity?
I am going to try this because I live in an area where the power goes nuts on a regular basis. I have 2 ST-70's one with a delay circuit and one without. It will be an excellent A-B test on which I will report back to the forum. Thanks again for the instructive commentary.
baddog

Thanks for your question. I was looking for a way to post up a JPEG scan of an altered schematic with the additional capacitance. No luck so far. If you or someone else points the way I will be glad to. A delay circuit is often very smart money. My I ask which one you are using?

dynacojoe wrote:Thanks for your question. I was looking for a way to post up a JPEG scan of an altered schematic with the additional capacitance. No luck so far. If you or someone else points the way I will be glad to. A delay circuit is often very smart money. My I ask which one you are using?

If you want to show a photo with your post, check the Stickys above on the main "basket" of the forum or just click the link below.

How to post a photo on the Dynaco Tube Audio Forum

Since this is a FREE forum no storage space for photos is allowed on the forum itself. You must host (store) the photo on some other web site. If you join > Photobucket (It's free) they allow free storage of photos and the amount of photos that you can store at no cost to you is pretty generous.

Bob

I would love to experiment with this "helper cap" upgrade! But if you have too much trouble posting the photo, maybe just a simple (non-technical) description of where the capacitor's two leads should be connected?

Thanks for the question. Your helper cap positive lead will need to go to quad cap lug "C". Lug "C" is the one located rear most and the only one to which a lead from the choke and a lead from each of the output transformers is connected. The negative lead ought to go to a suitably located twist-lock tab on the quad cap. Hope this has been a help. Soon I will give the photo of the schematic a try. Should go well.

Dynacojoe, thank you that's exactly what I needed to know!