6-6550 Tube Amp parallel push pull.

[gary350]

I found a lot of conflicting online information about 6550 tubes. Some claim 6L6 is a replacement, wrong pin out. Another place claims triode that is wrong. I looked in my 50 year old books, 6550 is a power pentode. Plate B+400v, Screen grid +225v, F 6.3v 1.6a. Load 3K. Plate dissipation 35W. Power Output 20W.

Does anyone know anything about this 50 year old amp or know how to find a circuit drawing? Don't need it but it would be nice.

After inspecting the wires I can see 3 tubes are in parallel, push pull. There has to be 6 primary B+ windings inside the transformer. There might be 6 taps for the screen grid too? Dynaco uses screen grid to get extra power output to the speaker. I have no clue how much extra power 6 screen grids add to total output.

Old caps have been replaced. Everything tests good. Tubes are original and good. Tested it on 200w speakers, WOW it will knock paint off the walls.

 

[crutschow]

That's quite an amp.
Can double as a room heater in winter.

Is there a name on the amp or is it home-made?

The screen grid connections may go to a tap on the output transformer.
It's called an ultra-linear connection and is used to reduce distortion, not increase power

 

[fourtytwo]

I can only see 8 wires on that trafo. Normally parralell anodes just drive a single primary, the same for screens so the primary should be just 5 wires leaving 3 for the secondary that is quite common for 16 ohm with an 8 ohm tap. Tracing the anodes & screens on the output bottles should reveal all.

 

[rjenkinsgb]

If the cab underneath it is part of it, then it looks rather like a guitar amp - is there any name or logo on anything, case or chassis?

 

[gary350]

I just learned this is a Fender PS400

 

[rjenkinsgb]

Here's the schematic:

https://schematicheaven.net/fenderamps/400ps.pdf

 

[fourtytwo]

Ahh not an ultra-linear then and custom selected valves, could be just for matching but also maybe some secret sauce to get more power out of them, for a short time........
I think I see 700V on the anodes,, we certainly used to crank up the volts on EL34's & 807's to get more out of them.

 

[crutschow]

we certainly used to crank up the volts on EL34's & 807's to get more out of them.

I'm remember seeing an operating tube power amp where the plates were glowing a low cherry red in the dark, obviously being operated close to their limits.
Tubes were EL34's or KT88's I think.

 

[Nigel Goodwin]

The screen grid connections may go to a tap on the output transformer.
It's called an ultra-linear connection and is used to reduce distortion, not increase power

It does both - but isn't usually used in guitar amps, as you're looking for distortion, not trying to avoid it.

 

[gary350]

There seems to be conflicting information online. Info says, there is a Fender PS300 and a Fender PS400 and several versions of each. Double checking tubes in the circuit drawing and the number on the amp, it appear to be a Fender PS400. This thing weighs 85 lbs on bathroom scales and online says it weighs 90 lbs. Need a fork lift to move it. LOL.

 

[crutschow]

It does both

Perhaps as compared to a triode, but not using the tube in a pure pentode configuration.
From Wikipedia:

• The power output is higher than from a triode, approaching that delivered by a pentode.

 

[gary350]

Data sheet says 6550 tube is 20w. How does 6 tubes = 435 watts output?

I always though 20w x 6 = 120w x 2 = 240w.

 

[Nigel Goodwin]

Data sheet says 6550 tube is 20w. How does 6 tubes = 435 watts output?

I always though 20w x 6 = 120w x 2 = 240w.

The valve specification isn't usually it's output power, it's the maximum power it can dissipate. However, on the sheet you posted above it specifies 35W dissipation, and two in push pull should be able to output considerably more than 70W.

The 20W is probably for low distortion, in single ended class A.

 

[gary350]

The valve specification isn't usually it's output power, it's the maximum power it can dissipate. However, on the sheet you posted above it specifies 35W dissipation, and two in push pull should be able to output considerably more than 70W.

The 20W is probably for low distortion, in single ended class A.

Dynaco use to sell kits, I wish I had a Dynaco 70 kit to build. Oh well.

 

[schmitt trigger]

Well....the schematic clearly mentions that the output tubes are specially designed and selected 6550 tubes. I imagine that one could push the plate dissipation significantly harder.

I also find amusing that on the Fender catalog it clearly advises NOT to use the amp on top of a speaker cabinet. Perhaps the higher plate dissipation and the significant vibration from the loudspeaker could cause the tubes to fail prematurely.

 

[Nigel Goodwin]

Well....the schematic clearly mentions that the output tubes are specially designed and selected 6550 tubes. I imagine that one could push the plate dissipation significantly harder.

I also find amusing that on the Fender catalog it clearly advises NOT to use the amp on top of a speaker cabinet. Perhaps the higher plate dissipation and the significant vibration from the loudspeaker could cause the tubes to fail prematurely.

It's a bass amp, so there would be a LOT of vibration, which valves don't like.

 

[gary350]

It's a bass amp, so there would be a LOT of vibration, which valves don't like.

I have been watching UK TV shows they are much better than USA rubbish. I have to learn UK English to understand what is being said. We say tubes, UK says valves. We park cars in a parking lot and UK says, Car Park. I think car park makes better sense than parking lot. In USA police arrest you but in UK you get nicked. What time is it, we say 5:30 and UK says ½ 5. We eat French fries and UK eat Chips. We eat potato chips & UK eat crispies. We say cookies, UK says biscuits. I still hear things I don't understand, I watch UK TV with captions on. We say TV and UK says tellie. UK TV is very entertaining I love it. American TV is, shooting, killing, hate, screaming, yelling, everyone is mad as hell, fast driving, car wrecks, shoot outs with the police, fires, fake explosions, no story at all just bloody rubbish TV. I only watch American TV to see local weather. I am getting a lot of enjoyment watching UK TV.

 

[Nigel Goodwin]

I have been watching UK TV shows they are much better than USA rubbish. I have to learn UK English to understand what is being said. We say tubes, UK says valves. We park cars in a parking lot and UK says, Car Park. I think car park makes better sense than parking lot. In USA police arrest you but in UK you get nicked. What time is it, we say 5:30 and UK says ½ 5. We eat French fries and UK eat Chips. We eat potato chips & UK eat crispies. We say cookies, UK says biscuits. I still hear things I don't understand, I watch UK TV with captions on. We say TV and UK says tellie. UK TV is very entertaining I love it. American TV is, shooting, killing, hate, screaming, yelling, everyone is mad as hell, fast driving, car wrecks, shoot outs with the police, fires, fake explosions, no story at all just bloody rubbish TV. I only watch American TV to see local weather. I am getting a lot of enjoyment watching UK TV.

You're a little out

We usually say 'get arrested' and it's 'crisps', we say TV, or television as much (if not more) than we say tellie.

The "you're nicked' thing was from an old 70's TV series called 'The Sweeny', and much of it (including the title) was from 'cockney rhyming slang' - something which is popularised in American culture. In reality, it's only spoken by a tiny number of people in one specific small area of London.

In general American actors doing English accents are absolutely abysmal, but presumably that is what the directors are looking for? - but it bears no resemblance to an English accent of any kind. Watching films we tend to think the actors are Australian, and wouldn't imagine they are supposed to be English.

There are some pretty strong English accents (such as Geordies, Scousers etc.), but I live in the North Midlands, where it's got a fairly neutral accent. When my daughter was at University someone commented that she, and a young lad from the next County, had nice clear voices with no accents - so they switched to talking 'broad Derbyshire' "Ay up, me duck!, t'rate or wot?"

 

[audioguru]

I laugh at the accent when they say oceans are full of wotah.
Their cars do not drive fast, instead they are fahst, Mate!
Aluminium? Pounds (of gold?).

 

[Nigel Goodwin]

I laugh at the accent when they say oceans are full of wotah.
Their cars do not drive fast, instead they are fahst, Mate!
Aluminium? Pounds (of gold?).

You've been watching too much American TV - as those 'accents' aren't English