Replacement Output Transformer for Fender Studio Bass...

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Replacement Output Transformer for Fender Studio Bass...

Postby jar240 » Wed Feb 04, 2009 7:54 pm

Well, this was a lucky find -- and an amp I'd never heard of before. It's basically a Fender Super Twin tweaked to make it suitable for bass.

Anyhow, the output transformer works fine but is for an 8-ohm load only. I'd like to be able to use it at 4 ohms without sacrificing bandwidth or power.

Would the Hammond 1650T or 1650TA be suitable for the design? It has a 1.9Kohm ct pp impedance.

Thanks,
Chris

Here's a schematic of the Studio Bass (links to a PDF):
Image
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Postby EWBrown » Thu Feb 05, 2009 6:16 am

It took a second look at the schematic for me to see that there are three pairs of 6L6GCs... I was wondering how they managed"200W RMS" from 6L6GCs Yellow_Light_Colorz_PDT_02

Presuming that the original OPT has a 1900 ohm primary, then the Hammond should work quite well, with a wider frequency response.

There may be a physical fit problem, as the Hammond OPT is most likely larger and heavier (14 pounds) than the original Fender trannie.

The 820 ohm NFB resistor will have to be reduced to around 560-600 ohms in order to work properly with 4 ohms output , but that is no big matter. The resistor would ideally be 820 ohms X 0.707 (580 ohms).
This can also be accomplished by connecting a 2K, 1/2 W resistor across the existing 820 ohm resistor.

This results in lowering the resistance to 581 ohms, which is more than close enough to preserve the original NFB characteristics.

/ed B
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Postby dhuebert » Thu Feb 05, 2009 8:47 am

Try it with the 4 ohm load, if you can live with the sound then don't change anything. If it's intolerable then, ya, start makin changes. 200 watts is a little large for 1650T but it is more likely to fit into the chassis than the 1650W which is more appropriately sized.

As I have said elsewhere; tubes are very bendy-stretchy and can tolerate a wide range of environments. You won't blow anything up with your 4 ohm load.

Don
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Postby Gingertube » Thu Feb 05, 2009 10:59 pm

Try Dons suggestion first.
If you then still think you need to change the tranny then the Hammond 1650T will do the job.

Based upon a resurrection job I did on a Super Twin some time ago the 6 off 6L6GC give around 165 Watts (Continuous Sine Wave into a dummy load) not the claimed 180W or the 200W listed above.

The Hammond 1650T is rated at 120 Watts 30Hz to 30kHz
At 165 Watts you'll only get down to about 40Hz instead of the 30Hz which you will notice not at all.

An Ozzie Yarn:
I bought the Super Twin cheap in a non working condition. One of my apprentices wanted something to blow away his brothers Marshall Stack (100W) so I got him to do the full restoration while I looked over his shoulder to make sure he got it right them sold it to him for what I paid for it plus the cost of the parts. We had to get the 2 x 12" speakers reconned as they were burned out.

He subsequently went to Alaska for 6 weeks for work and when he got back he found that the brother had borrowed it to play bass through - with the volume turned to 10 apparently. The brother confessed that flames had come out the back and it stopped working.
Some sleuthing showed that the braid connections from the speaker terminals to the voice coil had burst into flame and burned out.

Two replacement speakers and it came straight back to life - setting fire to its speakers did'nt worry the amp in the least.

Cheers,
Ian
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Postby jar240 » Fri Feb 06, 2009 7:19 am

Thanks for the suggestions everyone. Don, your pragmatic approach makes perfect sense but somehow eluded me! Sometimes it seems I buy tube amps not to play through but to tinker with -- flux smoke must be some kind of intoxicant... Yellow_Light_Colorz_PDT_11

The output appears to be less, subjectively, that I expected with pre and master on 10, active EQ flat. I tried my passive P-Bass and Jazz basses through an 8-ohm cab of around 101 dB efficiency. Any simple suggestions on how to increase the volume? I mean, I CAN'T get this thing distort using a passive input.

I replaced all the screen, grid and power resistors along with filter electrolytics since they were original and want to make sure it'll be reliable.

Thanks,
Chris
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Postby EWBrown » Fri Feb 06, 2009 8:35 am

Simple suggestion, though perhaps not th most elegant:

The T/M/B tone stack between the two sections of V1 eats up a lot of signal , if you don't need these three controls,
then bypass them, and you will have LOTS more gain.

Depends on if you need to "tailor" the output of the bass or not.


The easiest way to accomplish this is to cut the connection between the 0.0012 uF cap and the 330K resistor, and again at the "top" end of the volume control, then connect a 0.022 or 0.033 uF, 400V cap from the plate of V1B to the "top" of the volume control. You can leave the tone stack components in place, in case you want to re-connect them at some later time.

I suppose that this could be made switchable with a DPDT switch. It would be best to "hide" it inside, to avoid any unwanted loud "pops" if the switch is flipped when the amp is "hot".


The equalizer is still there if you need it.

HTH

/ed B
Last edited by EWBrown on Sat Feb 07, 2009 9:51 am, edited 2 times in total.
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Postby jar240 » Fri Feb 06, 2009 4:34 pm

EWBrown wrote:The T/M/B tone stack between the two sections of V1 eats up a lot of signal , if you don't need these three controls,
then bypass them, and you will have LOTS more gain.
Thanks Ed! I might replace input #2 with a switch to achieve this.

Actually, about the inputs... I don't quite understand the arrangement. Why do you suppose there's a connection from Input 2 to the grid of V1B, and that that connection is only in place if input 2 is empty? Also, I'm used to seeing 68K grid resistors in Fender front-ends, but here we have 33K. Also, any ideas why such a high value on the V1A cathode bypass cap?

Aside...I've got my basic amateur radio operators license but haven't been active in several years -- there's still so much to learn. I think I'm going to go to the next meeting to see if I can find anyone who can give me an oscilloscope crash course.

Again, thanks for all the info so far. This might not be one of the busiest forums on the 'Net, but the s/n ratio here is really high!

Chris
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Postby Mackortoyota » Fri Feb 06, 2009 9:09 pm

flux smoke must be some kind of intoxicant...

i set of fans to avoid the smoke, you actually inhale those fumes? :o
I believe solid state is the future, but only tubes should be entrusted with the task of audio amplification.
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Postby EWBrown » Sat Feb 07, 2009 9:32 am

I propose that a point could be made that the occasional inhalation of solder flux fumes may actually increase one's intelligence.

Yellow_Light_Colorz_PDT_03 Yellow_Light_Colorz_PDT_08 Yellow_Light_Colorz_PDT_04 :o Yellow_Light_Colorz_PDT_11 Yellow_Light_Colorz_PDT_01 Yellow_Light_Colorz_PDT_09

/ed B
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Postby EWBrown » Sat Feb 07, 2009 9:41 am

As far as the input jacks and their "odd" connection to the grid of V1B, it looks like the intention was to short the grid of V1B to ground, when there is nothing plugged into either input 1 or input 2. effectively a built-in "muting" circuit.

Which may also prevent that undesirable. loud 60Hz "death blast" when a cord is inserted. :o

(plug the cord into the axe first, then into the amp)

The 33K resistors serve as a very basic, non-adjustable mixer, when two guitars / basses are plugged in. That way guitar #2 won't load down the signal from guitar #1 (and vice versa).

If you plan to use one input only, then the 33K resistor could be eliminated, though this won't increase the gain by any noticeable amount.

See my uodated post about how to bypass the first tone stack - it isn't a good idea to put a "bypass" switch out front, where it can get "accidentally" flipped.

The 1 megohm resistor is the grid resistor for V1A.

/ed B
Last edited by EWBrown on Wed Feb 11, 2009 7:18 am, edited 1 time in total.
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Postby cartoonweirdo » Tue Feb 10, 2009 9:53 pm

I have never had a problem getting more gain out of Fender amps of any Year/model. Provided you like the sound, the secret for me has always been to play with the cathode and plate resistors in small doses throughout the amp rather than increasing one stage to make up for the others.

Not to toot the "blackface" horn here but I wonder if anyone has a comment on the driver. Looks pretty low gain to me.

Also in true late Fender style the bias is a "balance" rather than an "adjust" style. A lot of gain and some of the best tone available can be picked up here for some small modifications. This implies that you can stand playing bass that loud (should be fine unless you have a quiet drummer).

Carl
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Postby EWBrown » Wed Feb 11, 2009 7:14 am

The other way to increase the volume level, is to use more efficient speakers. I'm not too familiar with most MI speakers, and if they are high or medium efficiency.

How much LF SPL can you tolerate :o Yellow_Light_Colorz_PDT_11

Just watch out for 77 hz, I've read that it can do "funny things" to one's GI tract, and sphincter, with very high SPLs :o Yellow_Light_Colorz_PDT_04

/ed B
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Postby Gingertube » Thu Feb 12, 2009 9:40 pm

Carl,
The driver has plenty of gain. A "standard" 7025 (12AX7) Schmidt Splitter but with 220k Anode loads followed by 12AT7 cathode followers to drive the 3 output tubes per side.

Chris,
You said that you were using a passive pickup. I assume you mean a passive ceramic (piezo) pickup. If so, one mod you definitely should try is to increase that 1M resistor to ground at V1A grid. Try and 8M2 or even 10M.

On an amp I built for a friend I put in a switch to switch between 10M for a Fishman Bridge Ceramic Pickup on his old Gibson Acoustic and 1 M for his Fender Strat.

For more gain Ed's suggestion has merit.

If you don't want to mess with the amp at all then a JFET battery powered preamp to take the passive pickup output, provide the high impedance load for it and boost it to normal guitar levels would be the best option.
Something like this (but one with higher input impedance):
http://www.guitar-repairs.co.uk/jfet_guitar_preamp.htm

Edit:
A better link to a DIY passive pickup pramp.
http://www.scotthelmke.com/Mint-box-buffer.html

Cheers,
Ian
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Postby jonnyeye » Thu Feb 12, 2009 10:37 pm

If you increase the input resistor to ground to 10M, you'll be getting into grid-leak territory. Not bad, but it may give a different sound. Another option to increase the input impedance is to bootstrap the input: remove the 750uf cap, lift the ground connection of the 1M resistor and attach to it a 100k resistor to ground and 1uf NP (film? doesn't have to be high voltage) cap to the cathode of V1a. This will also probably change the sound (I've never actually done it nor heard it) as you're applying positive feedback. The most transparent way to increase impedance is almost certainly a FET buffer/preamp.
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Postby jar240 » Sat Feb 14, 2009 9:11 pm

Thanks for all the info... So far, I've been playing it with 8- and 4-ohm cabs, and it sounds plenty beefy. I'm think I'm going to wait on doing any mods since I think'll it work out for my gig situations just fine as-is.

Chris
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