Silverface Twin to Bass Amp Head?

[tbryanh]

OVERVIEW
I plan to convert a silver face twin reverb combo into a bass amp head.
(For schematic, see http://www.webphix.com/schematic%20h...om/fender.html)

OT ISSUES
Hammond 1750W's Frequency Response
The Hammond 1750W is a replacement OT for the twin.
Hammond Mfg. - REPLACEMENT & UPGRADES - Tube Guitar Amplifier - Output Transformers

The 1750W's frequency response starts dropping off at 70hz. So the transformer currently in the twin probably does the same. (See spec sheet http://www.hammondmfg.com/pdf/EDB1750W.pdf)

Hammond 1650TA's Frequency Response
I plan to replace the transformer with a Hammond 1650TA.
Hammond Mfg.-"Classic" Push-Pull-Easy Wire-Tube Output Transformers - (1608A-1620A, 1645A & 1650A Series)

The 1650TA's frequency response starts dropping off at 30hz, so it appears it will work good for bass.

Primary Impedances
The 1650TA's primary impedance is 1,900 ohm ct. This is close to the 1750W's primary impedance of 2,000 ohm ct. So it appears the 1650TA should work good in the twin.

Secondary Impedances
I plan to make all 3 secondary impedances of the 1650TA, 4-8-16, available on the back of the amp head.

NFB Loop
I plan to connect the NFB loop to the 4 ohm secondary tap of the 1650TA.

Footprints
The footprint of the 1650TA is much different than the footprint of the 1750W, so the 1650TA might need to be mounted to the head cabinet instead of the chassis.

Weights
The 1650TA weighs 14lbs, almost 3 times as much as the 1750W. The extra weight might be another reason not to mount the transformer to the chassis. The chassis may not be able to support the extra weight. (For the weight of the 1750W, see http://www.tubesandmore.com/scripts/...?item=P-T1750W)

CONNECTORS
If the OT is mounted to the head cabinet instead of the chassis, it appears it would be a good idea to use a connector to connect the OT to the chassis. What connector might be best for this?

OT CHOICES
Does it appear the 1650TA is a good choice? What other choices might be better?

PREAMP VOICING
What changes in the preamp voicing might be needed?

POWER SUPPLY
Do any changes to the power supply need to be made?

The B+ needs to be an AC ground to the audio signal. Will the current filtering capacitors work for the lower frequencies?

What about the PT? Will it work with the lower frequencies?

OTHER ISSUES
Are there any other issues?

[olddawg]

Just sell the amp and buy something more appropriate. You are setting yourself up for a lot of grief IMHO.

[loudthud]

Link to schematic not working for me... I looked at AB763 and AA769.

A Twin should work pretty good as-is because it has a lot in common with the Showman-Reverb. The blackface Showman was frequently used for bass before the silverface Bassman 100 was produced. Might be problematic with a 5 string bass though.

The extra low end of the 1650TA may not be needed depending on the speakers used and sound you are after. You could play around with the values in the tone stack or copy Bassman 100 or possibly Bassman 10. It's all a matter of taste. If you want to go with an Ampeg flavor, a baxandall like the Ampeg B15 might work for you. If you want to ditch the reverb and tremolo, Presence and Resonance controls would be a nice feature.

The filtering in the power supply could use a boost. Replace those 70uF at 350V with atleast 100uF, possibly higher.

[tbryanh]

I have a couple of twins I want to convert. They sound good with the various bass cabs I have.

My big complaint is that the 4 ohm output impedances of the twins does not give me the versatility I want.

Sometimes I want to run 1 amp with 1 cab, 1 amp with 2 cabs, 2 amps with 2 cabs, etc.

Upgrading to transformers that have 4 and 8 ohm secondary impedances will give me the versatility I want.

I figure while I am at it, I might as well improve the bass response too.

Upgrading the filter caps sounds like a good idea.

Does it look like the 1650TA will work (elecrically) in the twin (has the correct primary impedance)?

I don't think I can mount the 1650TA to the chassis, so I plan to mount it to the head cabinet and use a connector to disconnect it from the chassis when the chassis needs to be removed for service. What connector might be good for this?

[loudthud]

The 1650 primary impedance should be OK. I just looked at the only Twin I have, a Silverface without master volume, and it looks like the 1650T (not a TA) will just barely fit. ( I don't have a 1650TA to compare.) You might have to relocate the choke and/or it might interfere with the cap cover. I wouldn't turn it sideways, it might pickup hum from the PT. I would use one of the white nylon Molex connectors with crimp-on terminals and 0.156 inch pin spacing if you have to go that way. Use two, one for B+ and primary, one for the secondary with a different male/female pin configuration so someone won't mixup the plugs.

Are you sure you want to modify that vintage amp? Maybe you could trade for a Bassman 100. You would still have the 4/8 ohm problem though.

[tbryanh]

Will probably mount OT on head cabinet. Will probably need to run ground wire to ground OT. Will pay attention to orientation of OT to PT to avoid hum. Might hardwire instead of connector.

Mounting OTs to head cabinets will keep chassis's intact so twins can be restored if desired.

[J M Fahey]

I would use it as-is.
Great sound, crunch, balls, whatever.
Don't worry that much about a 30Hz monster transformer when *most* real world bass speaker cabinets (including the venerable Ampeg 8x10") start to roll off below 80 Hz, maybe having a bump "down there" to fatten things up a bit but no real flat response.
It *is* killer with any classic JBass, PBass or Rick.
If you need active basses, 5 or 6 strings, piezo pickups and the like, fine, but don't waste your time, go for the baddest preamp you like and drive some beefy PA rackmount power amp. ... and big loud boxes.
When you start "going down" efficiency dies and 100W become puny.
Use it as God (well, Leo) intended.
You'll love it.

[Gtr_tech]

A bassman 100 output transformer is a little beefier than the stock twin unit innit?