PT/OT matchup JTM45 Build

[TIMBO]

Hi guys, After reading this http://el34world.com/Forum/index.php?topic=21710.0 got me thinking of the PT and OT I was thinking of using in my next build.
I've got a pair of GEC KT66 (haven't had them tested yet) so of course I have to build a "Bluesbreaker" type amp.


In my tash I have a 300-0-300 PT and a OT that has three primary Z 10k, 6.6k and 5k @32w
I will be using a SS plug in the rectifier socket to boost the DC voltage but will try a valve.

Looking at the datasheet I'm not sure what I should aim for as a matchup,
http://www.ez260.com/kt66-gec-datasheet/
Or whether I go fixed or cathode bias. Thanks

[PRR]

We should know the current rating of the PT.

You have up to 420V DC.

Any of the larger tubes, 420V 6.6K 250r common cathode resistor, several-K screen resistor, will draw 100mA and deliver some over 20 Watts output and 20W Pdiss per tube. So I'd think about 5K load and 200r with 1K Rg2 to get 25W+ at around 24W Pdiss, around 125mA. 420V and 5K load fix-bias gets to 32W output, idle Pdiss whatever you want (suggest 15W-20W).

[TIMBO]

Thanks PRR, The PT current rating is 230mA.
I have got the circuit board just about done and have allowed for a fixed bias.
It will be a combo so around 30w will be great.

[mwelch55]

What program do you use to open your .sch file?  I have tried eagle and kicad and couldn't get it to open in either.

[kagliostro]

There are (at least) two programs that you can use

here info about where you can download it (and other info)

http://el34world.com/Forum/index.php?topic=590.0

---

EDIT:

Some time ago PRR give me this indication

Most of your power is in this winding:

290v-0-290v 1.15A winding

This can give 400V at maybe 0.6 Amps.

400V/0.6A= 677 ohms, the OT should be nominal 1353 ohms end to end.

So, you have 420V @ 0.23A

420/0.23A = 1826 ohm, the best match will be with a 3650 ohm load (the nearest value for your OT is 5K)

I hope PRR can say if this is correct

Franco

[TIMBO]

Thanks K.
The OT that I was going to use seems to have a problem
It was removed from a DIY that I bought on ebay and it sat on the shelf for sometime.
I marked the OT wires as it was in the DIY and at no time did I think there was anything wrong with it.
I was checking it to find the primary impedance and I was getting very low voltage readings that weren't calculating to the said primary impedances.

So I can only assume it's cactus 

I have another that has a 4K primary and if your calc's are right, this should work fine.

[Tony Bones]

Thanks K.
The OT that I was going to use seems to have a problem
It was removed from a DIY that I bought on ebay and it sat on the shelf for sometime.
I marked the OT wires as it was in the DIY and at no time did I think there was anything wrong with it.
I was checking it to find the primary impedance and I was getting very low voltage readings that weren't calculating to the said primary impedances.

So I can only assume it's cactus 

I have another that has a 4K primary and if your calc's are right, this should work fine.

It might be OK. Sometimes, if there's a lot of capacitive coupling between windings, measurements can look hinky. This seems to be especially true with transformers that have a lot of taps or multiple windings that can be connected in series / parallel.

Try connecting the CT of the primary to one end of the secondary winding. You can even ground this connection if you want. This won't affect your AC readings at all, but will shunt the capacitive coupling between primary and secondary.

[TIMBO]

Hi guys, some progress pics..........

[TIMBO]

Hi guys, some progress pics.





A couple more components to be added and hopefully some sounds later.

[TIMBO]

Hi guys, The amp has fired up well, BUT..........

The B+ has dropped dramatically with a SS plug rectifier
So I'm thinking the PT IS NOT rated at 230mAs as I had thought.
The PT was on the HOT side of warm, with the KT66 in, so maybe the added current of heaters and tubes is pushing the limits of the PT.
I have some 6L6s that i'll try and see how the PT handles the lower current draw.
The heater supply is also a bit strange, it has a CT but when measuring I get 1.5v on one side and 4.8v on the other.

[sluckey]

The B+ has dropped dramatically with a SS plug rectifier

What is the node A voltage with the power tubes pulled?

The heater supply is also a bit strange, it has a CT but when measuring I get 1.5v on one side and 4.8v on the other.

That's not very well centered. I'd probably call that a 0v - 5v - 6.3v. I would just tape off that 5v tap.

[TIMBO]

Hey sluckey, Your right about the heater supply.
So lifted the 5v tap and heatsinked, put in an artificial tap and get 6.8v across winding with no tubes in and 3.2v on both sides with tubes in.

With the KT66 in, node "A" measures 345v and tubes out 394v

With 6L6EH in, node "A" is 360v

[TIMBO]

Hi guys, Trying to think of some options for this build.
Sorted the heater supply and don't think it is the heaters that are heating up the PT.

I have two pairs of 6L6G and looking at there data, they look to draw a lower current at the B+ of the PT output.
http://www.r-type.org/pdfs/6l6g.pdf
So is it a better option to be in Class AB1?
OT is 4K

[PRR]

> look to draw a lower current at the B+ ... So is it a better option to be in Class AB1?   OT is 4K

B+ current is mostly about B+ voltage and the load impedance, _not_ the tube (if the tube is "big enough" to use what you give it.

You can NOT get into AB2 with any common guitar-amp driver.

[TIMBO]

Thanks PRR, Really don't what the next move is other than a PT that is of a known current rating.

[DummyLoad]

Thanks PRR, Really don't what the next move is other than a PT that is of a known current rating.

pair of 6V6 & double the load?


--pete

[TIMBO]

Thanks pete, Double the laod, as in 16ohm on 8ohm tap.

[DummyLoad]

Thanks pete, Double the laod, as in 16ohm on 8ohm tap.

yes, 8 on 4, 16 on 8. should get up around 375V with 6V6. power should be in the upper teens.

--pete

[TIMBO]

Thanks mate, I'll give it bash.

[TIMBO]

Hi guys, Tried the 6V6s and still finding the PT is getting very warm.

[DummyLoad]

how warm? you can hold a couple of fingers on it for a few seconds or you can't hold them on
on it at all? it just may be that is a poorly constructed part with high leakage that's causing it to run warm. the other aspect is that it's sized as such for a lot of small bottles (hence filament windig seems to be within spec) with a low(er) current HV winding.


what's the B+ sitting at with 6V6 and what's the bias set at?


--pete

[TIMBO]

360v

[Tony Bones]

To repeat pete's question: how hot?

If you can hold your hand on it for more than a few seconds then I wouldn't worry about it.

[TIMBO]

As hot as a drinkable cup of coffee

[Tony Bones]

That's too hot.

Just so we know, how hot does it get with no tubes plugged in?

[TIMBO]

Not a hint of warmth with no tubes in.
Put it on the light bulb limiter and don't even get a glow.

Digging through my stuff and found a 400-0-400 @150mA, and with a 5U4GB might get me a better result.

[DummyLoad]

Not a hint of warmth with no tubes in.
Put it on the light bulb limiter and don't even get a glow.

Digging through my stuff and found a 400-0-400 @150mA, and with a 5U4GB might get me a better result.

that'll get you upwards of about 550V unloaded or around 480V with 5U4GB moderately loaded, so 6V6 are out of the question. 

a pair of KT88 with SSR would work in auto bias, datasheets say 9k ra-a load with Pout of 50w: so double the load, your 4k is effectively an 8k load more or less depending on the speaker. should work. for B+ rail use stacked caps and cap center tap for g2 power. alternatively, you could use VVR or some such. if you run g2 at near B+ you'll need big swing from PI and 1K g2 resistors. use 100K grid leak for KT88 g1 ckt.

--pete

[TIMBO]

Thanks pete, I was afraid that upping the B+ I would have to increase primary impedance of the OT.

The OT is only rated at 40w.

[DummyLoad]

Thanks pete, I was afraid that upping the B+ I would have to increase primary impedance of the OT.

The OT is only rated at 40w.

best thing to do would be to order a 650-660VCT 200mA part.


--pete

[TIMBO]

Yeah..........................

[TIMBO]

Hi guys some progress here http://www.guitargear.net.au/discussion/index.php?topic=48531.0

Photo bucket has kicked the bucket