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V4BH with 6550's?

S

Samuel. K

Sep 12, 2007
22
1
#1
There was a really useful thread from October last year with regards to V-4B's with 6550's, but I'm after a more recent opinion. I'm thinking about getting my V4BH modded to accomodate 6550's. Has anyone had experience with this? I've heard some good stuff but nothing like 'i modded my V4BH and it sounds awesome' sorta.

So yeah. Anyone?
 
#4
If the amp is designed for the 6L6GC it may not be such a good idea to swap them out for a 6550. Apart from physical size the heater supply is very different. 6L6's draw 0.9A or 3.6A for four while 6550 need 1.5A each or 6A for four. The convertion will add just over 15VA continuously more drain on to the power transformer. While Ampeg transformers tend to be pretty substantial 15VA is quite a large increase in drain. You will shorten the life of the transformer.

The way to do the convertion properly is to add a heater transformer to the amp rated 6.3V @ 8-10A. Unwire the existing heater chain and re-wire it to supply only the small tubes. The new transformer should be wired to feed the 6550's only. The additional amperage will enable the new transformer to run below it's maximum specs for reliability.

You will also have to modify the negative bias supply to suit the 6550.

End result will be a different sounding amp certainly but a more powerfull amp no. Output power will be about the same as stock. You will not get 50% more power. The NZ amp may have been modified to give 150W, this BTW has not been proven and on the internet folks do tend to make 'interesting' claims. If the amp is using the stock output transformer it isn't going to last that long before it burns out.

Paul
 
#7
Paul, when switching from 6L6s to 6550s, would output power increase if replacing output transformer as well? Would be the additional heater transformer for 6550s needed in this case?
 
#13
I modded my Bassman 100 to use 6550's. I added a filament transformer for the new tubes, separate bias adjusts for each tube, and added a LOT of B+ filtering for extra headroom. I modded the bass channel into an Ampeg B15 type circuit and modded the normal channel into a London Power type preamp. And I added a resonance control.

The stock Bassman 100 wasn't enough for my gigging. It almost was a giggable amp (we used no PA support) but fell short. Not enough headroom.

After the mod to 6550's, it was most definitely giggable. Much more headroom, punch and a really solid low end. The Ampeg type preamp was really fine.

Since I didn't increase the B+ voltage, I don't expect I had any more power. But the power supply was stiffer and didn't sag, and the 6550's character was better for bass than the stock 6L6's.

I imagine 6550's in a V4B would be excellent. I would up the B+ filtering some. The sockets have to be rewired, but after rewiring for 6550's, you could switch back to 6L6's or EL34's with a bias adjustment. The bias supply may need to be modded to get the range you need for the 6550's.

I don't know how much current your power supply has. I added a filament transformer. I ran the new tubes from the new transformer which was rated at around double what the tubes needed. It fit into the Bassman head cabinet. I bolted it to the cap can cover. The preamp tubes and the PI were run off the stock supply.

I've modded 50 watt Bassmans to use a pair of 6550's and upped the B+ voltage. This made even the under powered Bassman solid and loud enough to handle many gigging situations. Customers were very happy. With a standard Bassman, room is limited. The chassis has to be removed to take the tubes out. I didn't need an auxillary filament transformer for that amp. The stock supply had enough, but we're only talking about a pair of them.
 
#14
Aye, but recall the OP was discussing the V4BH and not the original V4B. I would strongly suspect neither the PT or the OT on the newer version is anywhere near as stout as the original V4B models.

While a certain amount of additional B+ filtering can be beneficial in some cases, there soon comes a point of diminishing return; extra capacitance can't really compensate for a too-small PT with lots of sag.
 
#17
The power transformer is for transforming the current from the wall to current that can run the amp. The output transformer is for the amp to have any output to the speaker. The heater transformer supplies current to the heaters in the tubes (not all tube amps have one, though...mostly the bigger ones). B+ is high voltage DC power for the plates in the tubes.

That's the short versions, anyway.
 
#20
Whether solid state (transistors) or tube, many people do not realize this:

Tubes (or transistors) do not make power.

They merely use an input signal to modulate the power that is available from the power supply. If the power supply can only provide enough current and voltage to supply 100 watts, the amp can only "put out" 100 watts to the speaker (minus a little bit of power lost, since no amp is 100% efficient). You can put the biggest baddest tubes or transistors in the world in that amp, it's still subject to the limitations of the 100 watt power supply.
 

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