Question/help from Valve Amp Guru's

Hi Guys,

I'm restoring a '67 Fender Bassman I bought a year ago and have it recapped, retubed and biased, she is sounding great... esp. considering some of the ORGINAL MALLORY Filter caps were in there, 2 of them dead as doornails.

To the best of my knowledge/research the original output trafo on these amps, along with their BF/SF Bassman brethren want to see a 4 Ohm load, though my research says Fender OT's are great for impedance mismatching and are happy to push 100% of their rated output, quite comfortably driving an 8 Ohm load if needed.

But looking at the OT it didn't look original and so I thought it must be an after market unit... (more on that later).

So I decided to open up the Fender 2x12 cabinet to look at the speakers and imagine my surprise when firstly they are not only NOT the original speakers, but grossly mismatched in that they are both 8 Ohms, wired in SERIES, reflecting a 16 Ohm load!

Both speakers are also only rated at 25Watts, but I'm guessing that having them in series allows the 40W amp to drive them without blowing/burning something out, but being HALF as efficient at doing so.

That said I'm now trying to work out who made the aftermarket transformer and it's specs.



It has a logo on the side on it that looks like a black circle with a lightning bolt through it, with the following legending:

P-5417-1
7A18
549-6142

The primary side has a RED, BLACK and ORANGE wire and the SEC has BLACK & YELLOW+BLACK.

I measured the DC resistance across the primaries at 287R.

Given that info is there anyway to extrapolate who made this OT and/or what kind of load it wants to see (I'm wanting to replace the speakers and want to find a pair that match the amps output).

Thanks in advance!

Cheers

Matt

Hello Not sure about the tranny you have their but I have a original output same year to I think. you can have it Ive been meaning to use it built most likely never will so at least it can complete yours to spec if you want p.m me you would of course need to help out with shipping since I'm in the u.s.

This is the guy you wanna talk to

www.surrealamplification.com

his name is jerry, super cool guy!

If you are unsure of the transformer. I'd lift the connections and measure the ratio yourself if possible.

Otherwise, if it's cheap, just throw a re-issue trafo in there.

Matt

What is the valve lineup? Some of the early Bassmans were only rated at 45W, so the speakers could be fine.

It was only the later CBS silverface units that were boosted to 70W. However, there was a crossover period where the silverface units actually had blackface components inside 'em.

I have worked on a pair of blackface Deluxes that had similar looking output transformers to the one in the pic.

Will have to haul out the Pittman book & do some reading later......

Piet

Hey Roddy,

Indeed, going to have to test it myself with a nice 'little' ac on the secondaries and do some basic math...

Piet, config is 6L6's in a push pull config, 40W total power. It is a Blackface, pre CBS using the AB763 circuit, also found in some Deluxes.

The speakers should be fine in SERIES, but if I paralleled them it would give me 4 Ohms, but if each speaker is only 25W in parallel would that not make the TOTAL wattage 25W? In which case 40W into 25W sounds dangerous?

I need to get me the Pittman book, I just bought Dave Hunter's book and it is a GREAT read.

Thanks to all for the help, advice, and Loopers generous offer.

Cheers

Matt

The power is shared between the 2 speakers, so even in parallel, 2 x 25w drivers are fine.

Nice find....

Peter

peterc said:

The power is shared between the 2 speakers, so even in parallel, 2 x 25w drivers are fine.

Nice find....

Peter

Click to expand...

Thanks Oom,

Got to admit this is all kind of new to me, but a GREAT way to learn, I've learned alot form this rebuild and just learning about tubes, the book you sent me also helped, but a bit heady for daily reading.

I must say that impedance matching, speaker configs/ratings and the PLETHORA of ways to hook it all up is still taking me some time to get used to... I for some (irrational) reason thought that 25W paralleled speakers would divide the  RATING as well as resistance.

Cheers

Matt

2 x 25W speakers would be a 50W load in series or parallel.  An easy way to check what the transformer matches to the best is to have some 8ohm 50W resistors set up for a dummy load and then just make some quick measurements using 1khz and seeing what voltage and power you get into 4, 8, or 16 ohms.  The one that you get the most power into should be what the transformer is meant for. 

Getting a Mercury Magnetics replacement is never a bad idea either if you aren't happy with that one.

I think the deal that you mention about fender transformers being "good" for mismatching might come from the fact that the original Fender Bassman cabinet is wired for 8ohms even though the amp matches best into 4 ohms.  I think they expected you to run 2 cabinets making 4 ohms total. 

Have fun.

Here is a pic of it It came off a 67 bassman that someone put a mercury mag on.It's way more beefy than whats on their. What circuit is yours ab165?I usually convert that to aa864 although that upsets some purist it makes a lovely amp.(never mind saw earlier post) so is it a bassman or bandmaster?

Quote:"That said I'm now trying to work out who made the aftermarket transformer and it's specs."

For the first part of the question, here is a page which can be helpful when trying to date and identify a component:
http://www.triodeel.com/eiacode.htm

according to this the EIA code is : 549-6142
so this transformer would have been made by:Midwest Coil & Transformer
during the 42th week of 1961

Hope this helps
Axel

Hi Axel,

Thanks, great find!

Ok, now we know the manufacturer... lets work out the specs...

I hooked up an AC signal (+-30VAC to the primary) to the trano and found the following:

PRIM = 31.46VAC
SEC = 1.63VAC

Given the above logic says:

Turns Ratio = V in (to the primary) / V out ( measured in the secondary)

Therefore:

31.46/1.63 = 19.30 = 19:1 Turn Ratio

Impedance Ratio = Root of the Turn Ratio

19 x 19 = 361:1

Given that (and if my math is correct!) if the loads were as follows the Primary Impedance would be:

4 Ohm x 361 = 1.4K
8 Ohm x 361 = 2.8K
16 Ohm x 361 = 5.7K

According to the Fender specs the ORIGINAL Fender 125A6A OT had a 4.2K PRIM and 4 OHM Secondary.

I'm new at this, but would it then be safe to say given the specs above that the current transformer would actually be closer to the 4K source the OT was designed when driving a 16 Ohm load?

If so I can only assume that it is why the speaker cab has 2 x 8 Ohm speakers in series, making a 16 Ohm load?

Thoughts, ideas, random jeering?

Matt

Matt,

Those figures look good to me for a simple analysis. One thin I'd remember is that guitar amps can be quite crude in some respects.

The plate impedance of those 6L6s depends on how hot they are set to cook, but 5K is probably a good ballpark figure. With that in mind (and assuming it sounds good), I personally would run it as you suggested with the 16 Ohm setup.

Roddy

rodabod said:

Matt,

Those figures look good to me for a simple analysis. One thin I'd remember is that guitar amps can be quite crude in some respects.

The plate impedance of those 6L6s depends on how hot they are set to cook, but 5K is probably a good ballpark figure. With that in mind (and assuming it sounds good), I personally would run it as you suggested with the 16 Ohm setup.

Roddy

Click to expand...

Thanks Roddy.

Plates are sitting at around 420VDC, I think the MAX current for a 6L6 (or rather average, since it differs from company to company) is around 50mA, I've biased this pair at 35mA, which is around below 70%, which seems to be a fair 'average'... little like Goldilocks... not too hot, not too cold... just right, LOL.

The amp SOUNDS great, it could prob do with some better speakers, thinking of putting in the Jensen C12N reissues, though part of me wants to be daring and use 2 DIFFERENT 8 ohm speakers... seems a few manufactures are doing this like Dr. Z and Matchless.

Cheers

Matt

...

On a power amp like this it will be the most happy with the correct load.  The method I've always used, rather than trying to calculate much about the turns ratio etc, has been:  Hook up the amp to a load and change between 4, 8, 16ohms, the one that absorbs the most power is correct.  The amp is trying to transfer the power to the load and it will be most efficient at the one it was intended for.

On a tube amp, you must have a load on the output, if you don't you'll hear the transformer produce the tone and it will get hot and fail, because the power will all be sitting there instead of on the load. 

> I think the MAX current for a 6L6 (or rather average, since it differs from company to company) is around 50mA

That's idle current.

At 420V and 50mA the idle dissipation is 21 Watts. Original 6L6 was rated first 21W then 19W. 5881 replaces 6L6 with better reliability and 23W rating; this is when the BIG 2-6L6 amps arrived. 6L6GC is a TV tube sold for 6L6 uses and is nominally 30W dissipation. But with fixed-bias, the idle heat has nothing to do with the max-power condition. There is no compelling need to "use all the dissipation". In many cases, the "70% rule" works better than it should. 70% of 30W is 21W. If you feed 500V to the tube, then 42mA is a good goal. If you work at 300V then 70mA is quite fine.

You would also like your idle current to be around 1/4 of your peak current, so the crossover zone is smooth. But at typical supply voltages this leads to tube-melting idle. The classic BassMen run a bit rough around 1 Watt. This is arguably part of the "punch" of a Bassman, the sound leaps out as you lean into it.

> MAX current for a 6L6

With G2 near 400V, max plate current can be near 400mA on peaks.

With 400V plate supply you can swing almost 400V peak.

So the impedance to one tube may be as low as 400V/400mA= 1K.

You measured end-to-end(?), that's how we usually rate push-pull OTs, so the nominal primary impedance can be as low as 4K.

If plate and G2 voltages are different, you get different optimums. Original 6L6 was rated 250V on G2 and 360V on plate. This leads to more like 5K-6K optimum. You can even go to 550V (illegal but safe on most modern "6L6") plate and 300V G2 and get best power in 8K load.

Your OT appears to be a common 5.6K or 6.6K to 16 part, probably sold for hi-fi use. The power won't be much down from 4K, but the way it breaks-up is NOT the same as a true Bassman. The 16 impedance is awkward; if I only had one choice I'd rather it be lower. You could work it as 2.8K to 8, power won't be down that much from nominal 4K and now you do get the heavy-load grunt in overdrive.

I'm not sure what other differences there may be between Hi-Fi and Fender Bass OTs. A 5-dollar OT may claim 40W but sound very sad on the lowest octave. A hi-hi-fi OT will be too heavy and may sound "too clean" on the lowest octave.

If you were in the US, Germany, etc, I'd really suggest getting the 018343 stock Bassman replacement OT (or here); but shipping costs to your area may be daunting. (Then again, they come from China now, and maybe your local gitar-part distributor has a deal to throw a crate of parts overboard as the ship comes around the horn to get to Bremen.)

Fender OT 50 Watt 125A9A / 022855  2 Ohms 2.100 kg 69,00 EUR    
Fender OT 40 Watt 125A13A / 018343 4 Ohms 2.000 kg 89,00 EUR  US$75.00
Hammond 1750M Fender OT 125A13A / 022871 2, 4, 8 Ohm 3.500 kg 78,75 EUR    
Mercury Magnetics FBFBM-OM Blackface -- 4, 8 & 16 taps US$165.00

However if it seems to work fine, I'd turn to the speakers.

PRR,

As always, thanks you!

Sorry for going a bit quiet, wife and I went off the grid for a few days... was a great experience and just as well as my INET SP has been going through server troubles so wouldn't have had access anyways, LOL.

The amp doesn't sound BAD, but I'm sure it could sound better and since I've gone through all the effort or recapping with Sprague Atoms, retubing with JJ TESLA's and then biasing I may as well go the whole hog and replace the OT with something more akin to the original as well as better speakers.

Loopermc5 has kindly offered to part with his Bassman OT pictured below which seems to be a very suitable match, just need to figure out the cheapest way to get it here!

There is also a local amp maker who winds his own trannies and has been getting a rep from local players digging the sounds so I'll call him tomorrow and chat a bit with him as well.

Now... any thoughts on speakers? I know the originals would have been Jensen C12N's and or Utah's, they have mixed reviews, some love em, some hate em... any thoughts on a MIXED cab, maybe one alnico and one ceramic? Currently these 2 8 OHM speakers are different brands.

Again, thanks to all for your help!

Cheers

Matt