Fender Studio Bass OT?

[tbryanh]

The OT is burned up in the Studio Bass.

Mercury Magnetics has a replacement transformer (Mercury Magnetics -- The Guitar Amp Transformer Company), but I would like to use a Hammond "Classic" Audio
Push-Pull, Ultra-Linear Easy Wire - Tube Output transformer instead. (Hammond Mfg.-"Classic" Push-Pull-Easy Wire-Tube Output Transformers - (1608A-1620A, 1645A & 1650A Series))

Since the Fender Studio Bass is about 180 to 200 watts, it appears the only choice for the Hammond Easy Wire is the 1650WA.

According to this thread http://music-electronics-forum.com/t26091/ , if you want to put a Hammond Easy Wire into a Twin Reverb, the 1650TA is the correct choice.

It seems strange that the 1650WA and 1650TA have the same primary impedance, 1,900 ct.

Considering the Twin uses four 6l6GC's and the Studio Bass uses six 6l6GC's, it appears it might be wrong to use 1,900 ct for the Studio Bass.

Hammond says "How you operate the tubes (push-pull, push-pull parallel, ultra-linear, class, B+, bias, operating points, etc.) will change optimum plate to plate load impedance."

Will the 1650WA work well in the Studio Bass? (Is 1,900 ct an optimum plate to plate load impedance for the Studio Bass?)

If so, how might the tubes in the Studio Bass be operating differently than the tubes in the Twin that allows both amps to have the same optimum plate to plate load impedance?

[imaradiostar]

I'm I'm not mistaken you amp uses the same output transformer as a fender "super twin." The 1650wa is WAY overkill for that amp- I'm sure it would sound great but I'm also pretty sure it would dwarf the power transformer.

If it were mine to fix I'd find an output transformer that's capable of 100 watts from 20Hz to roughly 15kHz or more. It should be able to easily do 150 watts from 40hz (roughly low E on a bass) to 15kHz. I'd probably go with an Edcor because they're excellent quality, cheap, and available in a wide range of impedances. In fact I'd use THIS ONE.

That transformer would give you 4, 8 and 16 ohm taps. It would reflect 6.6k to each pair of 6L6's, right on the money for datasheet values. The other MAJOR advantage is that it has true 40% ultralinear taps rather than the mediocre 17% taps on the stock Fender OT.

If you have no use for a 16 ohm tap on a bass amp you could always use a higher impedance transformer and mismatch it. Use an Edcor with 3.3k primary impedance and use the 4 ohm tap as 2, the 8 ohm tap as 4 and the 16 ohm tap as 8. This would reflect 5k to each pair of 6L6's- once again, a fine value for 6L6's.

One other thing from my experiences with this output design. I'd make a few minor mods to improve the amp sonically and improve reliability. I'd increase the value (and wattage) of the screen grid resistors- probably to 1k, maybe 5 watt resistors. I'd also lift the negative feedback as the Edcor transformers with 40% taps will provide plenty of negative feedback on their own. I have a strong suspicion your ears will prove me right!

There are other brands you could try- Edcor is affordable and good quality so it's the first thing I thought of.

jamie

[Nickc]

Hi, i’m hoping I can possibly revive this thread. I’m a bass player and I have a fender super twin that someone gave me and I did a lot of work to recap it and rebuild rectifier circuit and replace blown resistors, etc. it’s a much healthier piece of gear now, but it still makes occasional gurgling farty sounds and I suspect the output transformer, so I’m hoping to replace it and also I have an old EV 15 L 8 ohm speaker that I want to use it with, so a new OT with an 8 ohm tap seems like good solution. Imagine my disappointment when the link you provided was broken! I’m also confused that you recommended 100 W transformer because the super twin is 180 W. You also mentioned output tube pairs, but the super twin has six. 6L6GC output tubes-2 sets of 3. I contacted edcor directly to see if they could help me, but first, they said that the super twin was a single ended output, which isn’t true, and now they don’t reply at all. Can anybody on here help??

[DrGonz78]

I don’t think so. Well maybe… don’t change the OT and the speaker at the same time. Perhaps it’s just the speaker making farts. Perhaps it’s farts with every speaker. But a fart is a fart. Fly back voltage hitting the OT from a nearly burnt speaker or bad speaker connection can make a fart when it gets loud. Peavey uses fast acting diodes to limit the current feeding the OT and they act like fuses as I recall. Burn open. The thread above says something about a burnt OT and it must be replaced. What is wrong with the OT on your amp. What is farting? Explain the fart in great detail

[DrGonz78]

I also remember working on an old Sunn tube bass amp. Complaint was similar with bad farts. I took the head and ran it at the shop with a different speaker. It was fine. I never bothered with the speaker cab so I open that huge behemoth. One speaker out of circuit measured 300ohms dc resistance. I think the speakers were tightened down badly and enough to cause warping. It is like the speaker cone is being pressed down on. Add in bass frequencies and it gets a lot of abuse. Not saying it’s your speakers or connections. But just have to rule that out.

[glebert]

I usually suspect everything before I suspect OT, if no other reason that it is one of the most expensive components to replace. When I've had bad OTs they were very low signal output across the board, not selective or occasional.

[Chuck H]

Hi, i’m hoping I can possibly revive this thread. I’m a bass player and I have a fender super twin that someone gave me and I did a lot of work to recap it and rebuild rectifier circuit and replace blown resistors, etc. it’s a much healthier piece of gear now, but it still makes occasional gurgling farty sounds and I suspect the output transformer, so I’m hoping to replace it and also I have an old EV 15 L 8 ohm speaker that I want to use it with, so a new OT with an 8 ohm tap seems like good solution. Imagine my disappointment when the link you provided was broken! I’m also confused that you recommended 100 W transformer because the super twin is 180 W. You also mentioned output tube pairs, but the super twin has six. 6L6GC output tubes-2 sets of 3. I contacted edcor directly to see if they could help me, but first, they said that the super twin was a single ended output, which isn’t true, and now they don’t reply at all. Can anybody on here help??

First... It's not a bass amp. That doesn't mean you can't play a bass through it of course. What sort of cabinet/load are you using? If you're using the stock speakers in the combo I don't think you can expect good bass performance. It's not likely the OT is "partially" bad (typically they work or are grossly bad). Unless the amp is underperforming for power there's no reason to change the OT or tubes. That amp has a typical Fender front end. This circuit is known for allowing too much bass adjustment for consistent performance across the spectrum. You may simply be asking the amp to make more LF than it can. I don't expect another OT could fix this. Please try this:

Plug the amp into a proper bass cabinet.

Make sure your master volume is all the way up.

Play the notes on your guitar that tend to cause the fartiness and adjust the bass control (or controls if you're using the graphic eq) as high as they can go without causing the fartiness.

That's what the amp can do. That amp isn't designed to behave at any tone setting, as some amps are. It's designed intentionally to allow over adjustment in some ways. This is so that if you have an input signal that is weak in LF you can still adjust the amp for more performance. The same amp settings that would be good for a weak LF input signal would be too much for a strong LF input signal.

I can't say about playing a bass through that amp, but on amps with the typical Fender front end most guitar players never set the bass control above four. Usually less.

[Helmholtz]

Schematic, please.

[Jazz P Bass]

The exact model was not indicated in the post.
Here are the 3 that I have.

[Chuck H]

The exact model was not indicated in the post.
Here are the 3 that I have.

I looked at the one schematic at Schematic Heaven. When I tried your link I got an empty folder.?.

[Helmholtz]

I suggest to start a separate thread.

[Nickc]

Thanks for the replies! Maybe My powers of description are as good as I hoped. The EV 15 L speaker sounds great when connected to an acoustic image class D amp and measures eight ohms. The fender super twin guitar amp is exactly the same circuit as the fender studio bass with some minor differences. The cap values in the EQ section are set to frequencies useful for guitar in the super twin, but I changed them to be useful for Bass. The super twin Cabinet and speakers are long gone. I found an old cabinet for a fender studio bass amp. I know some think it’s sacrilege, but I chopped it so that I would have a separate head and cabinet. The other difference between the super twin and the studio bass is that the OT in the studio Bass was 8 ohms instead of 4 ohms.

when I got the amplifier, it didn’t work at all. There were bad diodes in the rectifier circuit and various fried resistors. Once I got it going, there were lots of pops from arcing. I recapped the amplifier, I replaced all the sockets and plate resistors, and it has all new tubes, and has been biased correctly.


so it’s working great except for this one sound that happens every once in a while, which kind of sounds like someone turning a scratchy pot, but not quite. It’s sort of gurgly and bubbly. I’ll see if I can get a recording of it.

[Nickc]

Well, here it is running through a four ohm guitar speaker that I have. And it’s making a little bit of noise at the beginning of the video which I guess kind of sounds like static on your car radio. The other day, I was running it through a cabinet with 2 15s in it Which is about six ohms and in that case, the static sound starts to ramp up and volume, and then cut off and I remember now that it used to be followed by a pop from something arcing before I did all that work on it . So it seems like it’s not happy if I I give it something higher than a four ohm load. The master volume is on 10 in this video. And in the second video, you can see that the hum gets pretty bad unless the volume pot is at zero or 10. So I might’ve been giving myself too much of a pat on the back when I said that it was working great!

I wrote this post before I tried to upload videos and I keep getting an error message when I try to upload them so I guess you’ll just have to use your imagination.

[Nickc]

Just seeing the post about starting a new thread, which is a good idea. I will do that and also find a PDF of the schematic. Is there a protocol for uploading videos?

[Chuck H]

Is there a protocol for uploading videos?

There may be a time lag for new members for links and private messenging. I know that's how it was here for a long time. I don't know if that has changed. See below.