valve substitution

Downunderwonder · #1 03-23-2011, 04:31 AM

I just got a 50W amp which was designed to have 6l6 valves in it. Turns out to be running 7027A ones instead. Those were used in a 75W version of the same amp which had an upgraded transformer.

Good to go or gonna burn out the transformer?

Bassmec · #2 03-23-2011, 06:57 AM

If the power supply transformer has been upgraded to supply the higher plate voltage that tube uses to develop 30% higher emissions than the 6l6, Yes it would be a concern if the output transformer was not upgraded.
I don't think substituting a pair of 7027a tubes into an amp with max voltages designed for 6l6 is going to get more power, I think if its biased dead right with 7027a you might get the same output power with a slightly more mid forward tone, like you might expect from a 6ca7 or EL34FX2.
Make sure its biased properly and these tubes have not just been bunged in, it will be reliable assuming its a quality tube
set like RCA, GE, ETC.

Downunderwonder · #3 03-23-2011, 07:03 AM

They are JJ valves. Put in with new sockets, advertised as overhauled so I'd be fairly confident it has been biased.

It is very loud for what I imagine a mere 50W should give. It plays clean all the way to cranked.... if that gives any clues.

BUT...Looks like an original transformer for sure. You're saying without a bigger transfprmer it's going to get toasty?

Bassmec · #4 03-23-2011, 07:32 AM

Quote:

Originally Posted by Downunderwonder

They are JJ valves. Put in with new sockets, advertised as overhauled so I'd be fairly confident it has been biased.

It is very loud for what I imagine a mere 50W should give. It plays clean all the way to cranked.... if that gives any clues.

BUT...Looks like an original transformer for sure. You're saying without a bigger transfprmer it's going to get toasty?

No I'm saying that if nobodies put the mains transformer out of a higher powered amp in, without changing the lower power output transformer you should be fine.
Tubes don't make watts they don't have the voltages to drive.
Its a fifty watt amp, even if you put a huge pair of KT88's in it
and bias it beautifully, its still a fifty watt amp mate.
Its all in the transformers!.

Downunderwonder · #5 03-23-2011, 07:38 AM

Thanks, so it'll just be under driving those tubes? Would that be why it sounds clean when it's cranked?

Bassmec · #6 03-23-2011, 08:35 AM

Quote:

Originally Posted by Downunderwonder

Thanks, so it'll just be under driving those tubes? Would that be why it sounds clean when it's cranked?

Unusually Clean Loud Tube Amp?
It might be an ultra linear design amp with 43% taps in the output transformer, count the output transformer primaries
are there 5 or is it 3.
In the USA Fender bassman 70 was ultra linear so was the 135.
In British Amps The venerated Simms Watt amps where and so was the much worshiped Marshall Major.
Does your amp have a brand name and type, do you have a schematic?.
Yup a good fifty watt tube amp can seriously annoy the neibours
with well chosen, efficient cabs.

Downunderwonder · #7 03-23-2011, 08:49 AM

Jansen "Bassman" 50, not to be confused with the Fender of its outward inspiration, also no relation to Jensen.

http://www.ozvalveamps.org/jansen/bass50n1.jpg

Bassmec · #8 03-23-2011, 09:55 AM

Quote:

Originally Posted by Downunderwonder

Jansen "Bassman" 50, not to be confused with the Fender of its outward inspiration, also no relation to Jensen.

http://www.ozvalveamps.org/jansen/bass50n1.jpg

Its not an ultra linear design like the Fender Bassmann 70
its wired like an original Fender Bassman 50 in that respect.
The way Jansen get the higher power from your amp is to wack up the plate voltage on the ht winding on the mains transformer and use tubes that will stand it.
No mention of a different impedance/power, output transformer for the 75.

Downunderwonder · #9 03-23-2011, 10:40 AM

I don't know where that schematic originated. This one is a 50. I dunno if it's any louder than any other 50.

Here's a bit of a histology of the Jansen seriesJansen Your Specialist Professional Audio and Lighting Shop When I bought it I emailed the guy to hopefully get some advice on shipping it here. Asked about modding to the 75 spec at the same time. He said they don't stock the transformers to do that.

I sent another email on the valves but he hasn't come back to me so I thought I'd see what TB has to say about that in the meantime.

BassmanPaul · #10 03-23-2011, 11:15 AM

Not all that much left to say thanks to Bassmec!

While going from 50W to 75W is an increase in power of 50%, you actually need a power increase of ten times, 500W, to get twice as loud. The difference in power would be pretty well inaudible!

Downunderwonder · #11 03-23-2011, 11:30 AM

I know that, but I was interested in getting a more useful amount of completely clean power..... which it seems I may have acquired by accident, and now lack grind.

I'm happy if it's not going to cook a transformer or coke up a valve, whatever the technicalities of underdriving a tube are, I'm not up with them.

aborgman · #12 03-23-2011, 01:38 PM

Quote:

Originally Posted by Downunderwonder

I just got a 50W amp which was designed to have 6l6 valves in it. Turns out to be running 7027A ones instead. Those were used in a 75W version of the same amp which had an upgraded transformer.

Good to go or gonna burn out the transformer?

7027 and 6l6 are almost exactly the same:

Help, Uncle Ned, My Amp Has 7027's!

Given the same plate voltage, you aren't putting out anymore power.

BassmanPaul · #13 03-23-2011, 03:08 PM

Quote:

Originally Posted by aborgman

7027 and 6l6 are almost exactly the same:

Help, Uncle Ned, My Amp Has 7027's!

Given the same plate voltage, you aren't putting out anymore power.

IIRC, I don't have my RCA tube book handy, the big difference is that the 7027 has a different pinout on the base. I remember an amp I built for a friend who wanted to be able to swap out his outputs between 6l6GCs, EL34s and 6550s. I set up a switch to correctly bias for each type. What does he do? Sticks in a pair of 7027s from another of his amps and blows both of the screen resistors! It was a simple repair but still! Guitar players!

Selta · #14 03-23-2011, 03:10 PM

Quote:

Originally Posted by BassmanPaul

IIRC, I don't have my RCA tube book handy, the big difference is that the 7027 has a different pinout on the base. I remember an amp I built for a friend who wanted to be able to swap out his outputs between 6l6GCs, EL34s and 6550s. I set up a switch to correctly bias for each type. What does he do? Sticks in a pair of 7027s from another of his amps and blows both of the screen resistors! It was a simple repair but still! Guitar players!

Sorry, I had to LOL at this. That's great.

aborgman · #15 03-23-2011, 03:52 PM

Quote:

Originally Posted by BassmanPaul

IIRC, I don't have my RCA tube book handy, the big difference is that the 7027 has a different pinout on the base.

Slightly different...

...and whether or not it matters depends on the amp in question. For most Ampegs, it's a drop in replacement requiring no wiring changes.

Differences -

7027 - pins 5 & 6 are connected together, as are pins 1 & 4
6L6 - pin 6 missing, pin 1 not connected to anything.

To convert on non-Ampegs

1. Move any connections on Pin 6 , to Pin 5.
2. Move any connections on Pin 1 , to Pin 4.

7027 will still work if you make these socket wiring changes.

Downunderwonder · #16 03-23-2011, 09:36 PM

Well she goes good. I heard back from the Jansen guy, "all good". So thanks guys, all good.

If I'm reading that right a 7027 will always replace a 6l6 but a 6l6 may need socket rewiring to replace a 7027.

Mine are 7027A if that makes a difference.