I have a problem! (Don't we all?!)

[John T]

Hi,

I'm busy assembling parts for the get*set*go. However I've hit a problem! Here in the UK our mains supply is 240v 50cps AC. Unfortunately the specified mains trans is only available with a 120v AC primary.

I seem to have three alternatives:

1. Get a custom wound component from, say, Sowter - I shudder to think what that would cost!
2. Get a Hammond - excellent components but the nearest HT windings available are 325v 0 325v @ 150mA rather than 330v 0 330v @ 180mA. An additional 6.3v filament transformer will also be required (no great problem).
3. Finally, a friend has offered me a step-down transformer (240v AC > 120v AC) which I could use with the specified PA 774.

I'm sure I'm not the only person in the UK who who has built or is planning to build a GSG - so what did you use? Failing that - advice please from "the wise ones"

John.

[TomMcNally]

I think the stepdown would be easiest and cheapest.
The reason for the two 6.3 volt windings is to allow
DC on the filaments and cathode bias on each tube.
I like to keep everything balanced, so I wouldn't want
to use different transformers on each side.

Maybe Matt from Triode Electronics can talk his
transformer guys into coming up with a 240 volt
version of the PA-774 ?

[John T]

Hi Tom,

Many thanks for your prompt reply.

At the moment, I too am favouring the step-down alternative, primarily because, as I am using the PCB I don't want the hassle of fiddling to adjust the B+ when using a non-spec transformer - if I was wiring point-to-point I may have decided otherwise!

I've taken your advice and emailed Matt at Triode - I await his reply, but to be honest I'm not holding my breath!!

John.

[hembrook]

I posted a thread about 240V transformers below, but since no one reads them or replies to them, you likely missed it. It is very difficult to see, as it is titled "Power Supply Transformer for GSG" so I can understand why you missed it.

They cost me $83 a piece. The EI-Output transformers with 2.5k primaries and 4/8 ohm secondaries were $44 a piece. This does not include shipping, which even tho they are made in Serbia will have to come from the US. Customs and all that. No GST/VAT for you tho.

[John T]

I posted a thread about 240V transformers below, but since no one reads them or replies to them, you likely missed it......

Yes, I did see it! I've also heard from Matt at Triode and he tells me they may be producing a version of the PA774 with a 120 - 240v primary in maybe 4 months time.

I'm also awaiting a quote from Sowter. Their products are admittedly superb but usually pretty expensive - I suppose it's a case of "you only get what you pay for!"

[EWBrown]

I have one of the Trafomatic "ST35" toroidal power trannies and the OPT pair, obtained from Boris Sasic at Vista Audio Engineering (and Shannon has the other set). Yellow_Light_Colorz_PDT_03 .

I finally got around to setting up and testing it last night on an older DIYtube Rev B board.

The power trannie has a lot more capability than the stock PA774s, after two hours run time, it barely got warm.

The "gotcha" (there is always an unexpected "gotcha", it seems) is that it delivers excessively high B+ (well over 400VDC) with my local AC power which runs around 123VAC. EL84s won't last very long with that B+ level :o

I had to turn down the Variac to 113VAC output, in order to get a more reasonable 380VDC B+ at the OPT CTs. At that setting the filament voltage was 5.95VAC, but everything ran fine.

Since this is the G*S*G topic, I figure that this trannie might be just about perfect for a SS rectified 300B GSG design, as the 300B likes a higher B+ voltage, and 420VDC isn't excessive for that tube.

The "fun" part was figuring out the definitely non-standard color codes of the trannie leads, blue are teh two "hot" secondary HV leads, red is the CT, one filament pair is white, the other gray, and all four primary leads are orange. There are attached numbers on small plastic clips, but one still has to figure out the windings' phasing before proceeding too far.

The OPTs also have "weird" color coding, other than their B+ CT is red.

Of course, murphy's law struck, and on my first setup, the amp acted like a power oscillator, so I had to exchange the OPT primary connections to reverse the phasing. on the output side, yellow is Zero, white is 4 ohms and brown is 8 ohms.

FWIW, the blue (A1) and its associated SG black lead (2) connect to the plate and SG of the "front" EL84s, and the orange (A2) and its associated black SG lead connect to the "rear" tubes.

Once I got it all figured out, the amp sounded pretty good.

So my final approach will probably use the PT for a 300B GSG and use the OPTs and a PA774 or Hammond for an ST-35 design, or perhaps just try the set in a new design later on...

/ed B in NH

[hembrook]

I have 4 sets of Trafomatic iron on their way to me (someday...) so I will let you know. I posted the specs somewhere else, but I will be looking over the voltages as specified in the GSG manual and let y'all know.

I'll also take lots of pictures and see if I can dope out the color and numbering codes.

Cool.

Robert

[Shannon Parks]

The "gotcha" (there is always an unexpected "gotcha", it seems) is that it delivers excessively high B+ (well over 400VDC) with my local AC power which runs around 123VAC. EL84s won't last very long with that B+ level :o

What is 10%, right? Didn't a contemporaneous PP EL84 amp with the ST35 - like a Scott or Heathkit - have >400V B+? I would be tempted to just lower the cathode resistors in the DIY ST35, Ed. Maybe change the first filter resistor to a 100 ohm, 10W cement one. For my 6P3S ST35 I'll leave the voltage there. Maybe I should call it the ST40?

[EWBrown]

The 6P3Ss should be fat and happy with the higher B+, and I'd also SWAG around 20 WPC with 400-410V B+. THe Trafo-tranny runs fairly cool and since it is also 50Hz rated, it already has a 20% Volt-AMps rating "advantage".

My vintage foundling SA-100 runs its 7189s around 400V B+, with no ill effects. It is pentode mode, and fixed bias. THe OPTs are HUGE, and the primary is 6600 0hms CT. The SAMS photofact info shows a Stancor equivalent.

The original vintage 7189s were pretty "tired" from years of use, I replaced them with Russian 6P14P-EVs and it needed only a minor tweek for the bias to get it back on spec. The amp still needs some work, mostly re-capping and a good general cleaning to remove the years of accumulated dust and grime (it was run without the top cage or bottom plate) Fortunately no mouse "leavings" or fried dead bugs to contend with. :o
It still has the original 7247s and 5AR4/GZ34 rectifier, and they all tested as good so I saw no reason to replace them.

I'd definitely NOT use the J/J EL84ss in this amp, or anything with 400V or higher B+, they're just not as robust as the heftier members of the family...

/ed B in NH

[swelebny]

Any photos of the Trafomatic transformer built for the application?

For the guys across the pond contemplating step-down transformers, mounted on the chassis or separate?

Thanks!

Steve

[Ty_Bower]

The "gotcha" (there is always an unexpected "gotcha", it seems) is that it delivers excessively high B+ (well over 400VDC) with my local AC power which runs around 123VAC. EL84s won't last very long with that B+ level :o

What is 10%, right? Didn't a contemporaneous PP EL84 amp with the ST35 - like a Scott or Heathkit - have >400V B+?

My Rev B ST35 with the Tranco PT is still running over 400V most of the time. The Russian 6P14P-EV tubes don't seem to mind much at all.

And if they do cook? Who cares! It's only $16 for a new quad.

[EWBrown]

I tend to err on the side of caution most times, but sometimes, I'll go for broke and nice pretty glowing red plates Yellow_Light_Colorz_PDT_04 Yellow_Light_Colorz_PDT_08 :o

I'm still thinking of going "Rev D" with the trafomatic "hockep pucks" iron, I'll either have to get another new board, or else "gut" an existing amplifier (I already have too many of those sitting around the house) :o Yellow_Light_Colorz_PDT_06

Those Russian 6P14Ps and 6P14P-EVs seem to be almost indestructable.
I think that they weren't simply copying our 6BQ5s, they also had to engage in a little one-upmanship as well :o Make 'em MiG-25 compatible along with 6C33Cs and 6C41Cs...

Last time I got a batch of 16 of thee tubes, the total including shipping was $40. Ten bucks per refilll... Yellow_Light_Colorz_PDT_03

J/J EL84ss will probably go into red plate meltdown at 400V or higher B+, they're good tubes within their normal operating limits but they don't seem as robust as far as withstanding excessive over voltage / overcurrent abuses Yellow_Light_Colorz_PDT_07

I know that VTL gets 45W out of a quad of PPP EL84s in "pentode" mode in some of their amps, (Tiny Triode and the closely-related Manley Stingray) as they run the B+ around 420V, and keep the plate current down to 30 mA, so they are just about "firewalled" at those ratings.

For 240VAC line voltage users, it's best to mount the step-down trannie away from the chassis, some of these can throw a nasty AC magnetic field for a pretty good distance, especially the open-frame stepdowns.

/ed B in NH

[TerrySmith]

Seems like the supply of 6P14P's is drying up, you gotta look really hard to find them any more.

The Ram Labs/Music Reference RM-10 claims a rating of 35 watts rms with only a single pair of 6BQ5's, how is that possible?

[EWBrown]

That's called "creative specmanship".. Just like the magic formula which can power up a "500 Watt" set of computer speakers, using only a 12VDC, 1 Amp power brick.

Definitely it must involve a very liberal application of sooper-sekret Area 51 UFO technology...

Yellow_Light_Colorz_PDT_08 Yellow_Light_Colorz_PDT_08 Yellow_Light_Colorz_PDT_08 Yellow_Light_Colorz_PDT_08 Yellow_Light_Colorz_PDT_08 Yellow_Light_Colorz_PDT_08

/ed B in NH

[TomMcNally]

The Ram Labs/Music Reference RM-10 claims a rating of 35 watts rms with only a single pair of 6BQ5's, how is that possible?

I would think they are adding 17.5 and 17.5 and getting 35 ...

Ahhhhh .... that's why the Dynaco is called an ST-35 !!!