tweaking 6146b bass amp --doubling Power tubes plate current questions...

[LaszloS]

I have an amp i got from a friend it was built locally, it has at the moment two 6146b power amp tubes in push pull it is primarily a bass amp.  i am thinking about putting in four 6146b but am wondering about the plate current required to do this.

the data sheet shows a maximum plate current of 250ma for maximum signal two tubes in push pull AB1 600V plate, 200 screen. I have ~600V plate, 250 screen (i believe, will double check).

perhaps this is a dumb question but with four tubes does this double the plate current.  PT has a separate plate voltage winding that is rated to 350mA@600V.  will 4 tubes gunning melt this guy? 

the heater winding is sufficient at 7A

the OT is the hammond 1650R which is 5500 ohms CT it says can handle 100W, ~300mA a side so I'm thinking it might saturate a bit but should be OK

the data sheet shows around 80W for 2 in push pull so that would make 140W with 4? (dunno)

I don't have the schematic at the moment but it is roughly equivalent to http://www.triodeel.com/heathw6.gif

thanks very much for any help.

[jojokeo]

Sorry I can't give you the specific anwers to your questions since I'm on a laptop w/out data sheets, time, etc. but you are on the right track w/ your thinking through all of the various aspects & parameters. One or two things that stick out is the OT has the power available but the pri Z may be slightly off, but since it's a bass amp it may affect bass response and/or slight loss of power if not properly matched? (can't confirm at the moment w/out more info and a way to verify it). But you may want to consider this w/ your other considerations you are taking into account.
Another is that w/ all of the extreme parameters mentioned and/or planned - some of these extremes are only affected if you are running this amp 'full tilt'. Do you or your friend think you will be operating it to the extreme limits often or at all? This will be a big factor on the importance of getting everything as perfect as possible. HTH a little bit? J

[LaszloS]

awesome, thanks for your reply.  i suppose it depends on how loud it is running full tilt as to whether or not it will be played that way . i have to think probably not  the volume is pretty good right now but the bass is breaking up too early.

the OT has a 16,8,and 4 ohm tap so i could match up the impedance by running the 8 ohm cabinet into the 16 tap.

I can't help but think that this should work without melting my transformer (sonically I guess i'll have to wait and see but I am determined to go ahead if my trannies can handle it) for example looking at a twin reverb PT it states 310mA off the HT for the whole amp with a pair of 6l6s drawing 210mA at full tilt. a quad works there and i have 350mA for just the plate.  besides perhaps it will give it some character to starve it a little. the important thing is i really do not want to lose this transformer to the smoke.

[DummyLoad]

does this double the plate current

if you halve the load. yes.

your OT may not last. i'd opt for larger.

watch out for g1 ckt. minimum resistance. - i.e. the value of the grid-leak resistors of power tubes.

[PRR]

> four tubes does this double the plate current

If you keep the same load impedance: current and power hardly change. Maybe 10%.

To get the power, you double the tubes, half the load, double the current, get double the power.

Double power is NOT a lot louder.

Generally, once the power transformer is bought, the amp's power potential is what it is.

> data sheet shows around 80W for 2

Data for 600V 6K load shows 95 Watts. In 5.5K with a bit more than 200V on G2 it should be just about 100 Watts.

I think it is already as spot-on as any stage amp can be. There's no bolt-on tweak to increese power... to get "more power!" you are looking at a new engine-block and gearset. Which would be silly.... keep this as-is, get the 280W Hammond OT, a too-heavy PT, and a six-pack of 6550 on a new chassis.

Or build a 15W DeLuxe with big OT, and get a 600 Watt transistor-amp to boost it.

As ISO says, grid-leak on parallel large tubes becomes distressing.

[LaszloS]

thank for your counsels.

your OT may not last. i'd opt for larger.

i am surprised by this. hammond lists it on their site as good for 2 or 4 tubes with 310mA per side that is actually what got me thinking about this in the first place.

the amp is very loud and has gone head to head with a twin no problem but the bass is farting out. it doesn't need to be louder really what i would like is to tighten up the bass.  I have started tracing out the circuit and it could be that the power section is not where the problem lies.

just exploring possibilities.

I think it is already as spot-on as any stage amp can be. There's no bolt-on tweak to increese power... to get "more power!" you are looking at a new engine-block and gearset. Which would be silly.... keep this as-is, get the 280W Hammond OT, a too-heavy PT, and a six-pack of 6550 on a new chassis

yeah i would love to build an SVT clone but oh so expensive.

[RicharD]

>Farty bass...

1st off, rule out the speaker.  Next I'd look at the cathode bypass caps of the preamp stages.  A lot of folks copy Fender and use 15uF which I have found often causes fartiness.  Try a 2.2uF.