Too high B+ in 5F6A Weber Kit with M-M Iron

[Mark Black]

I posted this over at the Weber site forum too but feel more at home here...

Sorry this is little long...

A little background:

I've previously received a Weber 5F6A kit, assembled by someone and sold to the customer, for some troubleshooting with hum/buzz, etc. The build wasn't stellar, but after some tightening and looking after the grounding as well as some component mounting issues all was good. I thought it sounded pretty darned good too as a matter of fact. The filter cap setup is the small mini-board mounted inside the chassis. Rectifier tube is GZ34.

Fast-forward to the present:

Customer has brought the amp back for installation of all new iron - specifically Mercury Magnetics FO50BM-2 OT, FC-2.84 choke, and FTBP-58 PT. Aahh - the never-ending quest for tone.

Problem is the new PT & choke combination would exceed the voltage rating of the kit filter caps (20uF/450V on the extra board assembly) by a bit. Glad I used a variac, as I got to 445VDC at about 115VAC line voltage with all tubes in and biased reasonably close. Opening the standby switch pops the rectifier output up to no-load 470VDC, which of course is what the first stage filter caps will see momentarily when the standby switch is closed again. Actual line voltage around here ranges from 119VAC to 124VAC depending on time-of-day and/or time-of-year. I figure at 120VAC the first stage will be at about 465VDC with standby closed and something like 482VDC with it open. The choke measures only 35 ohms DC resistance and doesn't drop much into the next stage.

I notice the original 5F6A schematic shows 600V filter caps, but the 20uF Sprague Atoms currently available at that voltage are freakin' HUGE! I think I could fit some 100uF 350V caps in series on the little board to replace the two 20uF 450V, and I might be able to do the same with a couple 47uF 350V to replace the next stage 20uF 450V. I think the following stages will be OK after the dropping resistors.

I'm wondering - if faced with this kind of high B+ scenario what would some of you old hands do? Up the cap voltage rating? Or look at lowering the B+ through use of a different rectifier tube or zener diode in the center tap? Or some other possibility I haven't considered?

Thanks!

Mark

[Tom Phillips]

Mark,
Seems pretty much normal to me.
If you take a close look at the original Fender 5F6-A , the filter caps are labeled as “20-600P.” The “P” stands for peak. The actual caps used were rated at 500 Working Volts and it said that right on the part. The B+ usually runs between 440 and 460V and all the filter caps were down line from the standby switch so they all ran within their rating. It sounds like all the voltages you are reading are within what I would expect for the line voltages you give so you could just go with what you have. Or…you could use the center tap zeener diode mod to reduce your B+ a little.

On another issue, I understand about people experimenting with the OT but, if the original PT & choke were working OK, I don't really see the point in changing them. Just my humble opinion.
Tom

[MWJB]

Mark,

I would do as you suggest and replace the first & second stage caps with 2x350v caps in series. Depending on the size/brand of cap used at the PI you might be able to mount between board & buss.

Really you can save yourself a lot of potetial hassle & fiddling by mounting the filter caps in the doghjiouse on the back of the chassis in the first place.

The voltages you mention are not necessarily high for a 5F6A, I've seen as much as 486v on a '60, loaded.

[bob p]

i've used a lot of MM iron, and there's a definite disadvantage to using repro iron that is a 100% faithful reproduction of the orginal -- the line voltages today aren't what they used to be, and you always end up with overvoltage.

what would I do? if the customer were shopping, i wouldn't recommend the MM vintage power trannies. I'd recommend paying a little more for the MM PTs that are "modernized" so that you don't have to deal with over-voltage. but since that's not an option, the best approach is to get the line voltage under control using other methods.

i don't like to recommend a variac for the end user. if it gets turned up instead of down, the results could be disastrous. in cases like this, i'd install a bucking transformer in the amp (perhaps in the bottom of the cab) to throw away 10% of the line voltage.

[Mark Black]

Wow that was quick!

Thank you guys for your suggestions, they are just the kind of thing I need.

My background is more in fixing rather than building or modifying so I find myself in murky waters here, but I know enough to ask when I don't feel comfortable.

Thanks Tom for clarifying the original cap voltage ratings. I feel the same about the PT & choke, but this customer REALLY wanted them replaced. Then again we're talking about kit transformers here rather than replacing vintage components...

Thanks MWJB for the hint on the high side of normal operating voltages. Yes it would be roomier to use the cap can, but this kit was apparently built with the small internal board rather than the doghouse stuff, so none of it is there. I guess it's a Weber thing.

And thanks Bob for the background on M-M stuff. These are the first I've ever installed, and they came in with the amp, already chosen & bought without my involvement. Silly me - I did enough research to verify those part numbers were correct for the amp according to the M-M website and promptly stuffed them all in, cutting leads as needed. So 20% re-stocking plus shipping to change the PT out.

I shudder to think about a variac in the hands of most end users. Hell - I wouldn't trust mySELF to not make a mistake at a gig.

A bucking transformer is not something I immediately considered - I'll give that some thought.

Any of you guys (or anyone else for that matter) have an opinion about using a 5U4 rectifier in place of the GZ34? Patrick at M-M assures me the PT is conservatively rated to do 3A on the 5V winding so that could be an option and would introduce a little more voltage drop. Might even get me in the ballpark for the existing caps - I'll test tonight.

At any rate thanks again - I am looking to present the customer with a few options in case he has strong feelings about any one of them.

Mark

[bob p]

The bucking transformer thing is very easy to do. Essentially, you just need to find a 120VAC-12VAC transformer and wire it up properly to drop 10% of the input voltage. If you have 125 VAC at the wall, you'll end up with 125 - 12.5 = 112.5 going into the amp. That's almost perfect, and its Idiot Proof!

Get a transformer that has sufficient current rating to satisfy the amp and you're all set. IIRC RadioShack has 3A units hanging on the wall. Not sure about 5A units.

If you need some reference material, check out RG Keen's site at geofex.com. IIRC the write-up is called "the Vintage Voltage adapter."

[Chris / CMW amps]

I posted this over at the Weber site forum too but feel more at home here...

Sorry this is little long...

A little background:

I've previously received a Weber 5F6A kit, assembled by someone and sold to the customer, for some troubleshooting with hum/buzz, etc. The build wasn't stellar, but after some tightening and looking after the grounding as well as some component mounting issues all was good. I thought it sounded pretty darned good too as a matter of fact. The filter cap setup is the small mini-board mounted inside the chassis. Rectifier tube is GZ34.

Fast-forward to the present:

Customer has brought the amp back for installation of all new iron - specifically Mercury Magnetics FO50BM-2 OT, FC-2.84 choke, and FTBP-58 PT. Aahh - the never-ending quest for tone.

Problem is the new PT & choke combination would exceed the voltage rating of the kit filter caps (20uF/450V on the extra board assembly) by a bit. Glad I used a variac, as I got to 445VDC at about 115VAC line voltage with all tubes in and biased reasonably close. Opening the standby switch pops the rectifier output up to no-load 470VDC, which of course is what the first stage filter caps will see momentarily when the standby switch is closed again. Actual line voltage around here ranges from 119VAC to 124VAC depending on time-of-day and/or time-of-year. I figure at 120VAC the first stage will be at about 465VDC with standby closed and something like 482VDC with it open. The choke measures only 35 ohms DC resistance and doesn't drop much into the next stage.

I notice the original 5F6A schematic shows 600V filter caps, but the 20uF Sprague Atoms currently available at that voltage are freakin' HUGE! I think I could fit some 100uF 350V caps in series on the little board to replace the two 20uF 450V, and I might be able to do the same with a couple 47uF 350V to replace the next stage 20uF 450V. I think the following stages will be OK after the dropping resistors.

I'm wondering - if faced with this kind of high B+ scenario what would some of you old hands do? Up the cap voltage rating? Or look at lowering the B+ through use of a different rectifier tube or zener diode in the center tap? Or some other possibility I haven't considered?

Thanks!

Mark

www.tubeampdoctor.com in Germany does sell excellent 20u/550V caps (I can also supply them if needed). Otherwise use two >40u caps in series with some 220k Rs in parallel.
The 230/240 Volt PTs from MM I did use over here for tweed Bassman amps did give about 440 - 450 Volts B+...

[MWJB]

I would be careful with the bucking transformer approach, if your heater voltages drop below 6.2VAC it's time to put the thinking cap back on. Some people say that low heater voltages, within limits, are no bad thing...personally I don't agree, they can adversely affect dynamics.

In the first instance certainly try a 5U4, see how it affects the envelope, lots of guys use them in 5F6As anyway.

Alternatively, for a significant B+ drop, the reverse polarity 50W zener diode trick works well. A stud mount device, requires drilling a hole in the chassis.

Another course of action, for a more subtle voltage drop might be to sub out the 6L6s for KT66s, you might lose 15-20v with the Reflector/GT HP/Harma KT66s. NewSensor have a new "KT66" that they sell as a "drop in replacement", I don't know if this tube would have the same effect. The KT66s have a little less high end chimeyness, but again some guys love them in 5F6As.

At the end of the day though, if your B+ is between 450v & 475v you have authentic 5F6A voltages.

[Mark Black]

OK thanks everybody again for your input.

Here's the wrap-up:

I went with series-wired caps in the first two filter stages. The caps were a very tight fit on the little cap board that came with the kit, but I got them on there with a little hookup re-work.

At this point the amp was pretty much done - sounded good, voltages reasonable.

But the new higher voltages aggravated what had been a minor problem of trace DC on the inputs of the bright channel, causing bad noises when rotating the volume pot on a guitar. The amount had gone from trace to about 250mV. Eventually I figured out that even this brand new eyelet board was just conductive enough to be the culprit. Moving all the input resistors off the board and directly onto the jacks cured it.

And that's a wrap!

[bob p]

I would be careful with the bucking transformer approach, if your heater voltages drop below 6.2VAC it's time to put the thinking cap back on. Some people say that low heater voltages, within limits, are no bad thing...personally I don't agree, they can adversely affect dynamics.

Yes, I would not recommend that anyone try the bucking transformer approach unless they have the problem that the bucking transformer is designed to cure: an amp with a 110 VAC primary that is being forced to run on 120+ VAC mains. If that's the case, the heaters will also be overvoltage, signficantly above 6.3 VAC. If the bucking transformer brings the effective line voltage for the amp back down to the design parameter of 110 VAC, there can't be undervoltage on the heaters. If there is, then the amp has a different problem -- like the wrong PT for the application or a mis-wound PT whose turns ratio on one of the windings is out of spec. If something as unlikely as that were to happen, the answer to the problem would be to have Mercury replace the transformer.

IME the Mercury Magnetics tweed replacement PTs are exactly what they're supposed to be. They're designed so that if someone builds a Bassman using their repro iron, they'll end up with an amp that is just as overvoltage on a 120 VAC main as a vintage Bassman!

[bob p]

Eventually I figured out that even this brand new eyelet board was just conductive enough to be the culprit. Moving all the input resistors off the board and directly onto the jacks cured it.

And that's a wrap!

congrats on the fix. i bet that was a real head-scratcher.