bassman circuit question(s)

[zhyla]

Hi, I'm pretty new to amp building. I built a little Firefly last year and I'm itching to do another amp, but learn a bit more in the process. I have an EE background but not specifically in circuits and amplifiers.

I'm trying to come up with a BOM for a widepanel (5B6) Bassman but need a little help. Here's the (only) schematic and layout that I found:

Schematic: http://www.ampwares.com/ffg/schem/bassman_5b6_schem.gif
Layout: http://www.cnjradio.net/fenderamps/b...5b6_layout.gif

I understand the circuit from a high level (preamp, treble cut, phase inverter, power amp). My main question is on the transformers. I think a Hammond 125E will work for the OT (correct me if I'm wrong), but I'm really fuzzy on what kind of parameters the PT needs to have.

According to the layout, the output of the rectifier is 400 V. I did some back of the envelope simulations and think to rectify 120 VAC into 400 VDC you need a transformer with a ratio of 1:5.5 or so. I haven't been able to set up a SPICE sim yet to verify this. Assuming that is correct, where do I go from there?

Another minor question is that if the supply is really 400 VDC, 450 V caps is kind of cutting it close isn't it?

I've got all kinds of questions, but I won't drop them all at once.

[MWJB]

The OT should have a primary Z of around 4K, you'll probably see approx 60-70mA at the tube's plates, with a 250ohm cathode resistor, so the OT should be rated for a little over that.

330-0-330V to 340-0-340VAC B+ secondary winding should get you in the ballpark B+ with a 5U4 rectifier. 200mA current rating would be good, 4A minimum 6.3VAC heater current capacity.

50v headroom between B+ & cap ratings will be fine, you will probably find 500v caps easily enough.

Just out of interest - this is an unusual version of the bassman to go for, what made you pick this one?

[zhyla]

Thanks so much for the response. How did you come up with the range for the PT secondary winding? Is there a way to calculate that or is it just common knowledge? Also, I'm considering doing a SS rectifier, will the same PT work?

Looks like I'll be needing a bigger OT, glad you mentioned the plate current. How did you arrive at this? 320V/250ohm = 1.28 A, but that's assuming no internal resistance in the tube. I'm fuzzy on how to pull this from the tube data sheet ( http://lib.store.yahoo.net/lib/thetubestore/ts5881.pdf). There's a lot of graphs of current vs plate voltage, but I think I need to understand the biasing of the power tubes first to make sense of it all.

Anyways, I chose this version because... it's a little simpler than the subsequent versions and it's something you can't buy a reissue of off the shelf. I figured I'd learn a bit more than just assembling a 5F6-A. I've never played this version so who knows what it will sound like.

[MWJB]

"How did you come up with the range for the PT secondary winding? Is there a way to calculate that or is it just common knowledge?" Well experience really. You can get a ball park idea by taking the rating of the B+ secondary at one leg, then multiply by the rectifier you are using, then subtract a little to account for current load,...

A solid state rectifier will multiply your B winding by a factor of 1.4, so maybe you would want a little less on the B+ winding 310-0-310 to 320-0-320VAC?

HOWEVER - there is a degree of guess work, depending on whether mfr's specs are accurate, hoew the amp is biased, brand of tubes, how your wall voltage relates to PT taps available, wall voltage at the actual time of day you measure voltages (it fluctuates) etc. So to get with a margin of +/- 20v would be reasonable.

If you go high, you can reduce plate voltage by running a reverse polarity zener in series with th B+ centre tap. If you go low, you might be able to find a stiffer rectifier (not the case if you go SS rectified) or rebias/try a different brand of tube to push voltages up?

"How did you arrive at this? 320V/250ohm = 1.28 A, but that's assuming no internal resistance in the tube. I'm fuzzy on how to pull this from the tube data sheet" Well experience again, that 320V will become significantly more in DC when rectified with diodes & filtered, but you know that B+ dc voltage should be around 390v at the tube plate, you can therefore expect 60mA+ at the 6L6plates, overdesign a little to save burning up the PT.

"Anyways, I chose this version because... it's a little simpler than the subsequent versions and it's something you can't buy a reissue of off the shelf. I figured I'd learn a bit more than just assembling a 5F6-A. I've never played this version so who knows what it will sound like." Well it's entirely your choice but it raises a couple of questions? Why build an amp that you don't have a sonic refernece for...you may hate it? If you don't know what to expect, how will you know if it sounds right on completion? There are a lot of things that can trip up a novice builder, you wouldn't be the first to build a malfunctioning amp and attribute the result to "quirks of the circuit".

I'd strongly suggest building something that you have a datum to refer to as regards the sound, then you'll be assured of an amp that sounds like what you want it too, & if it doesn't - you know that you have some investigating to do.

[Bruce / Mission Amps]

If you want a simple and not high powered version, simply use the the smaller 6L6 power and output trannys for a silver face Pro Reverb... been there, done that and it works really well with a 5U4GB rectifier.
Many after market vendors sell these transformers.

I'd use around 340vac to 360vac and around 150ma to 180ma PT for a more vintage sound.
The OT can be any +25 watt OT from 4000 to 6500 ohms but don't use a Hammond 125E... they are just not big enough or will sound good enough for a pair of 5881s.

[Enzo]

The cathode resistor will not have the B+ across it - the tube is not a short circuit. The current through the tube will drop a voltage across that resistor and that establishes bias for the tube.

SOlid state rectifiers drop a half a volt across the diode. Tube rectifiers drop tens of volts across themselves, so with a particular power transformer, a solid state rectifier might provide say 400vDC, and a tube rectifier on the same winding might yield only 350-370 volts DC into the first filter.

Get a copy of the RCA tube manual or other tube text, and read on basic tube function. The tube when in a circuit with B+ applied to plate and cathode grounded would conduct as hard as it could if left alone - it would be a diode. Introduce a grid, and the grid, when more negative than the cathode, interferes with the current flow. It controls it. Negative enough and the grid will cut off the tube completely. We establish bias so the tube is partially conducting at rest. We set that point so it falls in the center of the curves, so the signal can move either way. A tube works sorta like a FET.

[zhyla]

Everybody, thanks for the advice. I've got hold of a copy of the RCA RC-30 receiver tube manual and starting at the front. Partical physics from the 50's, great stuff.

It's true, having never heard this amp I may not like it. If so I'll have to either tweak it or tear it down and build something else. Either way I learn something.

Bruce, could you elaborate on the relationship between PT current rating and the "vintageness" of the sound? Were older PT's underpowered?

Do you guys have personal preferences on tube vs SS rectification? I don't have any real reason to do SS other than it saves a few bucks and it's simpler to wire up.

[MWJB]

"Do you guys have personal preferences on tube vs SS rectification? I don't have any real reason to do SS other than it saves a few bucks and it's simpler to wire up." If in doubt, wire up a tube socket as if you were going to run a tube. Then you can install a tube, or wire some diodes into a tube base/buy an SS plug in rectifier & swap between them. It's not really any more work to wire up for a tube. When changing your rectifier make sure to check your bias (google tube biasing) current, or set plate current with the SS rectifier then you know you won't burn anything up when switching to a tube.

If you were really concerned about cost, you wouldn't be building your own amp. :-)

To determine PT B+ current rating add together plate currents for the power tubes, max grid current for each power tube, double the result and add say 10mA per preamp tube & a little over if you like to keep everything running stable.

So you might have 60mA+60mA plate current, 10mA + 10mA screen current, 10mA + 10mA per preamp tube = 160mA, so Bruce's suggestion of 150-180mA is spot on for an authentic tone. The amp we are discussing is cathode biased so we are not expecting plate current to rise appreciably in use.

Some Fenders had just enough current handling, some had plenty to spare. Some were designed with the right amount, but as wall AC voltages rose over time, headroom current handling has diminished (more B+ = plate current, without rebiasing). Vintage Fenders actually run voltages 10-15% higher nowadays, than those specified in the schematics.

If your current demands on the PT are excessive your B+ voltage will fluctuate wildly with adjustments to bias current, in extreme cases your heater voltages may be pulled down, your PT may even burn up. A big draw on any of the PT secondaries will have a 'knock on' effect on the remaining secondaries. That's why I prefer to have a little excess in hand, the PT will run more evenly, cooler and you may have the option to experiment with tubes with a higher heater current draw.

Do the same for the 6.3VAC heaters: 2x.9A for the 6L6s, 2x .3A for the preamps = 2.4A, again add a margin to ensure stable running & longevity, 4-5A should be fine.

A SS rectifier in an amp with a PT with a 340-350VAC B+ secondary will usually give around 460-480vdc after the rectifier, so decide on the rectifier before the PT. This kind of voltage will be murder on the tubes if you go SS rectified with the stock 250ohm cathode resistor.

By the way make that 25uf bypass cap at the power tube cathodes 50-100v rating as you'll have over 25v here.

[zhyla]

Well I got this old organ amp to chop up for parts. It's got a massive power transformer, 6L6 output tubes, a 5U4G rectifier, a choke, and a voltage regulator tube. I pulled all the tubes except the rectifier and powered it up briefly to see what the B+ voltage was - 480V at the rectifier output.

Now my project (schematic, layout) needs 400V. Should I expect 480V to come down a bit when everything is plugged in?

If I need to drop the B+ by 80V or so, how should I do this? I see this done with resistors in amp supplies all the time. This design drops the 400V to 320V and 300V, for example. I've not yet understood how that is done - can someone straighten me out?

Someone previously mentioned you can use a zener diode to do this also. How?

[MWJB]

"Should I expect 480V to come down a bit when everything is plugged in?" Yes, it will come down, at the moment there is no current load on the PT, voltages are artificially high. Voltages are measured under load.

"If I need to drop the B+ by 80V or so, how should I do this? I see this done with resistors in amp supplies all the time." You probably won't need to, cross that bridge as & when you come to it. You typically don't drop main B+ voltage in amps with resistor, a reverse polarity zener diode between PT centre tap & ground is a better method.