What's the maximum power for a 5f6-a clone

[Jester]

Hi Guys,
A new amp builder here, with a little bit of a worry. I've bought in a bunch of recommended items, from various suppliers, to build my first Bassman clone I'm starting with a 5f6-a cct. and I have a bit of a concern that my power supply might pump out to many volts. Please correct me if I'm wrong (as I do wish to learn). The original design had a 325 vac outputs to the GZ34 recitifier. I assume this means the DC power is 325* root 2 - losses of the rectifier. ie 325*1.41 - 26v = 432 VDC. The MM Axion FTBO-DB+ power transformer that I have has a low voltage output of 380VAC, I therefore assume I'm going to get 510VDC out of the rectifier. Having viewed a number of other posts here on this site, I'm a very concerned that this is too high for my TungSol 5881, or the EH 6L6GC that I have to try in this amp.

My assumption is that I will have little loss from the centre tap on my output transformer (another MM transformer - Axiom FTBO-MM) and therefore be putting about 510VDC straight onto the plates of my output tubes.

This appears to be higher than recommended for these tubes - if I don't fry them, I understand I might get lots of headroom on the amp, and I'd be very happy with that. So here are my questions:

1. Has anyone tried this PT on a 5f6-a clone, and had success?
2. Do I need to worry about this level of voltage or not?
3. If this voltage is considered too high, can you please advise me of the best method of reducing this down to a safe level (using the current transformer - I understand paying out a few hundred bucks more would solve the problem but....)?
4. Finally, I'd like to make a mod to the original design, by using 2*220uf/350 v cap.s in series (with 220k ohm, 1 watt resistors in parallel) as the main filter caps, and I intend to install these on the rectifier side of the standby switch. Is this safe if I intend to use the GZ34 tube rectifier, rather than a solid state device?

Thank you for your help with this.

[tubeswell]

First place to start with questions about Tube plate and screen voltages is the relevant tube datasheet. Have a look at some 6L6 datasheets.

Current draw is the other main thing to watch. The PT secondary windings have to handle the current draw of the tubes you are using. The 5V recto winding might be 2A or 3A or 4A rated, and that will determine which rectifier(s) is/are safe to use with it. Again refer to the relevant tube datasheet(s).

The rectifier tubes themselves are rated to supply a certain amount of current to the B+ rail. The combination of output and pre-amp tubes you are using needs to draw less than whatever this rating is, otherwise you could damage your rectifier.

Also the pre-amp and output tube current is drawn via the recto tube from the high voltage winding, so that also has to be rated for the right amount of current draw, otherwise you will damage your PT. Current underatedness is generally a bad thing in this regard. (i.e. you really should have more current rating than you need, and I'm not just talking about the idle current - you need to make a minimum of 50% allowance above the summed idle current of all the pre-amp and output tubes -other people might tell you to have even more of a buffer).

[MWJB]

1 - not specifically tried the MM PT you describe but have tried Hammond 274BX and found an out of the box RI 5F6A that ran 514vdc on the plates. An original 5F6A will typically run 470vdc-ish (+/- 10vdc) on the 6L6/5881 plates when biased up, with a GZ34. If your Tung Sols are reissues then they should take this kind of voltage - many late 50's/early 60's onwards 6L6 Fenders (plus numerous current production amps) run final voltages in this region, it makes little sense to produce a current production 6L6/5881 that can't take a little over 500vdc. If your Tung Sols are New Jersey NOS...well it's your call, they'll be pricey to replace.

2. Not from a reliability point of view, no.

3. If you did want to drop B+ voltage for reasons of tone, fit a 50W, 30-50V reverse polarity zener diode between B+ centre tap & ground. Or, fit a tweed bassman style PT with a 345-0-345VAC secondary.

4. Yes, this will be perfectly safe.

[Jester]

Thanks Tubeswell for your answers and comments below. Very useful to a beginner like me.

First place to start with questions about Tube plate and screen voltages is the relevant tube datasheet. Have a look at some 6L6 datasheets.

It is because I've read a few of these that I'm worried/ concerned that I'm not fully understanding what I read. The max. plate voltage shown on the RCA data sheet (which I believe is from 1960) for the 6L6GC appears to be 500V, while the Tung-Sol data sheet (dated 1962) shows the max voltage for the 5881 to be 400v. I note on the RCA data sheet, it claims that the 6L6GC is the same rating as the 5881.

My understanding, looking at the cct diagram, and some rough calculations leads me to conclude that the 5881 should not be used in the Bassman - I'm sure you see why I'm concerned that it's my understanding of what I am reading, as the 5881 appears to be the original tube.


Current draw is the other main thing to watch. The PT secondary windings have to handle the current draw of the tubes you are using. The 5V recto winding might be 2A or 3A or 4A rated, and that will determine which rectifier(s) is/are safe to use with it. Again refer to the relevant tube datasheet(s).

Thank you for this - I have 1/2 checked this, and the PT I have will allow 7amps on the 6.3v winding, and 4amps for the 5v winding - so easily meets the first requirements - the spec sheet does not specify the allowable current for the main B+, so I'll find out from the manufacture.

The rectifier tubes themselves are rated to supply a certain amount of current to the B+ rail. The combination of output and pre-amp tubes you are using needs to draw less than whatever this rating is, otherwise you could damage your rectifier.

Also the pre-amp and output tube current is drawn via the recto tube from the high voltage winding, so that also has to be rated for the right amount of current draw, otherwise you will damage your PT. Current underatedness is generally a bad thing in this regard. (i.e. you really should have more current rating than you need, and I'm not just talking about the idle current - you need to make a minimum of 50% allowance above the summed idle current of all the pre-amp and output tubes -other people might tell you to have even more of a buffer).

Thank you for this response, I will check I have enough current - my guess is I'll have it, and some. Just like the voltage - I think this might be the top of the range PT for the application, unlike a lot of the early designs which I believe were underrated for financial reasons.

[Jester]

MWJB,
Thanks so much for the detailed response. It's been very useful.
[/COLOR]

1 - not specifically tried the MM PT you describe but have tried Hammond 274BX and found an out of the box RI 5F6A that ran 514vdc on the plates. An original 5F6A will typically run 470vdc-ish (+/- 10vdc) on the 6L6/5881 plates when biased up, with a GZ34. If your Tung Sols are reissues then they should take this kind of voltage - many late 50's/early 60's onwards 6L6 Fenders (plus numerous current production amps) run final voltages in this region, it makes little sense to produce a current production 6L6/5881 that can't take a little over 500vdc. If your Tung Sols are New Jersey NOS...well it's your call, they'll be pricey to replace.

Thanks for sharing this knowledge - I feel a lot happier to know that not only the originals ran at higher voltages than I thought, but that you've acually tried it with another PT.

2. Not from a reliability point of view, no.

Okay, that's good news.


3. If you did want to drop B+ voltage for reasons of tone, fit a 50W, 30-50V reverse polarity zener diode between B+ centre tap & ground. Or, fit a tweed bassman style PT with a 345-0-345VAC secondary.

Call me Mr Cautious, but I think I'll give this a go. I'll buy a handful of different voltage rated 50W zener diodes, and try voltages between the two extremes until I find the level / tone that I like.


4. Yes, this will be perfectly safe.

Brilliant. Thank you so much from one very happy amature amp builder...

[Jester]

Finished the build

Hi Guys,

I just thought I'd let you know I've finished the build, and started testing out the cct. With only the GZ34 in place, I get a whopping 548VDC on pin 8, with the mains transformer putting out 397 VAC on pins 4 & 6.

I'm sure you'll notice that I have not yet received the revise polarity zener diode, which I'm told will be with me in a few days.....

So as a quick pole, has anyone run Electro Harmonic 6L6GC, or new TungSol 5881 at 550Volts? Let's get some feedback, has anyone gone much over 510Volts on the plates of these tubes? If not, I'll continue to wait for the zener, but I'm eager to try the new amp....

[tubeswell]

Hi Guys,

I just thought I'd let you know I've finished the build, and started testing out the cct. With only the GZ34 in place, I get a whopping 548VDC on pin 8, with the mains transformer putting out 397 VAC on pins 4 & 6.

I presume you have only plugged in the rectifier and not plugged in any of the other tubes. When you plug in the other tubes you'll find the voltage on the B+ should drop quite a bit because of the load.

You can always plug then in + rig up a DC voltmeter from the plate to ground of one of the 6L6s, and have your hand on the standby/mains switch if you see them starting to red-plate.

[Jester]

Yes, that's right I've only got the rectifier tube in at the moment, and I am just testing out the cct, to make sure I connected it up correctly and putting the volts in the right place.

This is only my 4th build, so I still have a lot to learn (and will probably look back at these questions in a few years and think oh what a %ick). So far I have only seen a small drop off in power to the plate voltages of the power tubes when installing the other tubes. I am assuming this is because there is little resistance between the rectifier and the plates here and I have expensive (a strong pt), obviously when the current starts to flow in the cct, I get voltage drop down the power line (across each of the resistors) to the other tubes in the cct, but I'm assuming putting in these other tubes will not have a large (40Volt) effect on the plate voltage of the power tubes. Also because the cathodes are both grounded, I don't at the moment see how the bias will drop the voltage across the tube - so I'm still a little concerned.

What's the maximum voltage you've put on a 6L6? Have you 'beaten' MWJBs 514 Volts? Or has anyone else?

The tone I like is generally clean, so working my way up to a high voltage will be my goal - I guess I'm just looking to see how far above spec. people run with these tubes and still have success.

Thanks for your response. If my Zener diode does not turn up soon, I'll try your suggestion....

[MWJB]

525vdc on 6L6 plates is not unheard of with some vintage Fenders, a pal of mine has been running a pair of Sovtek 5881s at 525v for several years now. The Sovtek 5881 is not everyone's first choice tone-wise but sounds great in some amps and can take the voltage happily.

[Jester]

Thanks MWJB, that put you in the lead - Anyone want to put in a higher bid???

I'm now hoping that my copper losses in the transformer are as Tubeswell suggested and a bit higher than I was expecting yesterday, then I get down there without the zener diode.

[Jester]

Hi Tubeswell / MWJB

I just wanted to let you know that I powered up the bassman clone yesterday. The mains power was down a bit, so in standby the B+ voltage was only 534VDC. With the confidence you both gave me from your experience, I decided to flip the switch and see if the rest of the amp worked. To my surprise (but not to Tubeswells) my B+ voltage dropped to 482VDC (well, I guess a more accurate description would be, it settled at that after I biased the amp to give 35ma per 6L6). This leads me to believe I have copper losses of about 52 volts in the PT or about 500ohms (so it hasn't taken me years to think oh what a %ick - but days ). Initially I had a few crackles and pops, and a low frequency jack hammer sound, but it soon settled out and now sounds fine.

So I just wanted to say thanks again for your help - I think I would have been to nervous to try powering it up with out some of your valuable experience. I've now got another working amp - Thanks guys.

Jester

[tubeswell]

FWIW I don't have much experience either ;-)