B+ Too High?

Hey y'all. My patient is an early Mark II that's been rebuilt to Mark III specs and running a VTA Octal driver board. Perhaps some of you remember my saga of getting the stupid thing to work again after rebuild. It's up and running now and I've been listening to it for a couple hours per day for the last week or so. It sounds fine, stable, doesn't seem to be overheating, however B+ is about 25-30VDC over specs (exactly 500VDC B+, 410VDC going to the board) using a 5U4GB rectifier with all tubes installed and biased.

Mainly just curious if it's safe to leave as-is, or what the options are if not. I mean, I enjoy working on these things but at some point I'd like to just enjoy it, ya know?

Thanks as always for this wonderful resource.

Some basics:

Do check your wallplate voltage. These days, it is commonly higher than once-upon-a-time.

The amp was designed against +/- 115 VAC at the wallplate.
Taking a common example of 123 VAC at the wallplate:  115/475 = 123/508
Given that the 5U4 has a bit more voltage drop than the 5AR4 that could get you to 500 V.
Or not-quite 123 VAC at the wallplate.

On Safety: If the transformers are not excessively hot, there is no particular life-safety issue going on. However, tube life will be shortened somewhat to considerably.

On dealing with it:  There are any of several easily achieved options:

a) A true RMS line-conditioner - not the cheapest option, but one that is set-and-forget on your part. I would define this as the idea solution as it adapts to varying wallplate voltages.
b) Permanent Bucking Transformer - available on eBay, and may be home-made reasonably easily. This bucks wallplate voltage by a fixed amount, and with the understanding that operationally, you might be able to run safely from about 110 VAC (actual) to 120 VAC (actual), a 5V buck ought to be enough. In my mind this is the second-best solution as unless you are in an area with wild fluctuations, it should do.
c) Installation of a variable autotransformer that you adjust adjust as needed. This does solve the problem, and has some flexibility. I am "not so sure" of this solution as I am an unashamed Variac Snob - and will not have a piece of off-shore junque as a key item in my system. If one uses a good-quality device of more-than-adequate capacity, go for it!

Wall voltage is about 120-121 (it was fluctuating a few .10's +/- from 120.4).
I thank you for the suggestions and I'll look in to them.

*Edited to remove a question about just using a variac. I see you addressed that in your last comment.

If you just want to use as is with no variac or other device then there are a few things to think about...

I also use my MK IIs with the 5U4 rectifier. It is a directly heated tube so at turn on the B+ will peak much higher than the 500 VDC once the output section conducts and settles in. I have measured 560+VDC for several seconds. I had two failures of the quad caps due to what I think was an over voltage at turn on. I replaced those caps with the 550 VDC version that have a 600 volt surge ratings. Either the Authenticap or the Dynaparts.com caps will have those ratings. Look closely at each rating though. No problems after that. All other components should/will be within acceptable tolerances on their ratings.

Although within their ratings the higher voltage does cut into what safety margins are available for each of the reaming components. Using a 5AR4 in lieu of the 5U4 will allow a slower B+ turn on with lower surge peak but it will also have less total voltage drop and the overall B+ may be higher yet. Turning the bias down on the output tubes does help give them longer life. Not much else you can do except to follow Peter W recommendations to lower line voltage but using some device between the wall and you amps.

I just plug them into the wall and just accept the higher stress on the amps. No other problems so far.

so at turn on the B+ will peak much higher than the 500 VDC once the output section conducts and settles in.

A rectifier tube does not pass B+ until it heats. This is not instantaneous.
However, it may overload the filter caps until the output tubes heat as well.
Usually the differences is 2-5 seconds.
An effective solution to this issue would be a Time-Delay relay - which high-wallplate voltage or not is a good idea.

We are blessed with pretty constant WP voltage of 118 VAC. Were it any higher, I would invest in a bucking transformer for the tube equipment, and a line-conditioner for the SS equipment as due to amp size, those systems draw a bunch more current (potentially) than the tube systems.

The bottom line is that changing main filter caps is a genuine PITA, and if some small investment(s) will reduce that need, it will be money or time well spent.

The 5U4 does take a few seconds to heat up but it is many seconds before the output tubes fully conduct. The total time for the overvoltage maybe in the 6-10 second range. Still classifies as a surge to me.

I replace the quad caps anyway on my amps as I do not trust 50 year old ones. I do check them and and many sections will still have reasonable low ESR. I just am uncomfortable with the internal condition of the paste, spacing and overall health of the cap. They have to go.

drafalske wrote:Hey y'all.  My patient is an early Mark II that's been rebuilt to Mark III specs and running a VTA Octal driver board.  Perhaps some of you remember my saga of getting the stupid thing to work again after rebuild.  It's up and running now and I've been listening to it for a couple hours per day for the last week or so.  It sounds fine, stable, doesn't seem to be overheating, however B+ is about 25-30VDC over specs (exactly 500VDC B+, 410VDC going to the board) using a 5U4GB rectifier with all tubes installed and biased.  

Mainly just curious if it's safe to leave as-is, or what the options are if not.  I mean, I enjoy working on these things but at some point I'd like to just enjoy it, ya know?

Thanks as always for this wonderful resource.


 

My wall voltage can get up to as high as 125-126Vac so I have it on a variac until I can build that Bucking circuit to keep it around 117-118Vac.

Peter W. wrote:so at turn on the B+ will peak much higher than the 500 VDC once the output section conducts and settles in.

A rectifier tube does not pass B+ until it heats. This is not instantaneous.
However, it may overload the filter caps until the output tubes heat as well.
Usually the differences is 2-5 seconds.
An effective solution to this issue would be a Time-Delay relay - which high-wallplate voltage or not is a good idea.

We are blessed with pretty constant WP voltage of 118 VAC. Were it any higher, I would invest in a bucking transformer for the tube equipment, and a line-conditioner for the SS equipment as due to amp size, those systems draw a bunch more current (potentially) than the tube systems.

The bottom line is that changing main filter caps is a genuine PITA, and if some small investment(s) will reduce that need, it will be money or time well spent.

I corresponded with Bob on the time delay relay and tube rectifiers. We both agreed that the time delay relay should only be used with solid state rectifiers. Using the TDR with a tube rectifier will subject the tube to hot switching every time the amp is turned on. This hot switching will expose the tube to excessive currents as it is suddenly exposed to an empty filter capacitor when the relay closes. Just wanted to pass this on.