O.T. for a 4x6L6GC amp

[rzenc]

I need to spec a 120W push-pull for a badass 4x6L6GC amp.
I went to check tube data sheets and the closiest specs I found were at G.E./R.C.A. sheets- voltage wise.
They claim 55W per pair according to these conditions.
O.T. primary should be capable to take 300mA's per side? How about primary impedance? To a pair they suggest 5600 ohms plate to plate impedance, If I'm going with a quad Am I supposed to half it, say 2800 ohms of impedance between plates?  It's secondary will have taps for 4-8-16 ohms.
Does these numbers looks ok??

Thanks in Advance.
Best Regards,
Rzenc

[stingray_65]

I checked the primary Impedance spec on a 4x 6L6 OT I have here and it is 2.1k

It is supposed to be a replacement for these Fender OT's :125A29A, 022889, 022899, 037612

Not much more useful info on the spec sheet.

Ray

[DummyLoad]

Around 2k Ra-a will work. actually, 1.7k to 5k will work, depends on your bias point. you're on the right track though, take a 50W plan - double the tube count, half the load.  

But 100W Fender Twin or 100W Marshall are flooded on marketplace, is it for guitar, bass, or are you leaning more to HI-FI frequency response?

.... lots of 100W iron on the market... :-)    

http://www.edcorusa.com/category/67-cxppseries.aspx?wattage=37&primary%28input%29impedance=265&secondary%28output%29impedance=279

Hammond 1650RA w/ 2-4-8 ohm load...

Hammond 1650TA

Plitron http://plitron.com/wp-content/uploads/file/2100CFB.pdf

Triode Electronics. Dynaclone 100W will work - 4K3 pri - half the load run Fender Twin PT

Best bet -  just buy Doug's Fender Twin PT/OT set for paltry 180 clams and get blindingly fast delivery...  

[Tiny_Daddy]

I would get one of those replacement transformers for Marshall JCM800, 100 Watt.
Doug sells them.

[rzenc]

I would get one of those replacement transformers for Marshall JCM800, 100 Watt.
Doug sells them.

Unfortunately, to ship such piece of iron overseas would cost me an arm and a leg... however I found a shoppe which provides me great iron - P.T's, O.T's, chokes.... - for as much as you pay on a piece of hammond iron in the US and I don't need to worry about shipping and custom taxes. Actually custom taxes sucks so bad down here that it will ended up doubling the whole package value, for instance, If I buy things that are US$ 100,00 then there is shipping, say US$50,00. When it arrives on the customs they will charge 100% on the final package price, that is 100% on US$150,00. In other words, I will pay another US$ 150,00 just because I decided to buy it from other country...

Around 2k Ra-a will work. actually, 1.7k to 5k will work, depends on your bias point. you're on the right track though, take a 50W plan - double the tube count, half the load.   
But 100W Fender Twin or 100W Marshall are flooded on marketplace, is it for guitar, bass, or are you leaning more to HI-FI frequency response?

This guitar amp is supposed to be loud and clean, with strong lows. The main purpose of this amp is to play at big venues, probably with 2 4x12" cabs - the player is a great friend and he plays with a big shot from the national country scene - I'm in brazil ok.. not the US 
I would like to know about current capacity, is 300mA's DC per side on the edge here? According to data sheet, each side will be dragging around 230mA'sDC on peaks of full roar, how much would be a safe factor to extrapolate???

Many thanks!!!!!!

Best Regards,
Rzenc

[HotBluePlates]

According to data sheet, each side will be dragging around 230mA'sDC on peaks of full roar, how much would be a safe factor to extrapolate???

Talk to your transformer winder. Let them help you decide. But think through the whole process first.

To a pair they suggest 5600 ohms plate to plate impedance, If I'm going with a quad Am I supposed to half it, say 2800 ohms of impedance between plates?

Yes, that's the right approach. Can you be sure that you will use 450v plate and 400v screen as suggested in the sheet? If you think you will be using a typical Fender-style power supply, that screen voltage may be nearer the plate voltage of 450v. You will want a choke for the screen supply node, as the sheet mentions that the screen current at full power is 4 times the idle value.

You were thinking, "Same supply voltage, double the output tubes, half the OT impedance, and that gives double the power." This is correct. But remember: same voltage, half the impedance also means double the current. If 210mA was max-power current for 2 tubes, then it will be 420mA for 4 tubes.

But screen voltage may go up to 450v to make power supply design easier, and that would increase tube currents, which are already at 87% of max dissipation at idle. Be sure to give yourself a good range of bias voltage available to dial the current down (say -35v to -55v), and to allow for tubes which need more bias voltage to cool them off.

And only promise 100w; you might get 110w if everything is perfect, but say it's 100w and let the extra be a bonus for the customer. It keeps you from having to work hard

[PRR]

IMHO, 120 tested Watts from four 6L6GC in stage-amp duty is risky. I'd use four good EL34, six 6L6, or two four 6550.

With 4K-5K6 loading you want 450V-500V supply.

The condition you cite (BTW that only says 110W/pair, though there's more if you allow more THD and ignore G2 dissipation) draws 232mA at 450V, 104.4 Watts DC for 55 Watts audio or 52.7% plate efficiency. That's typical of larger tube amps. You hope for 120W out, you need 228 Watts of DC. At 450V-500V this is 506mA-456mA DC demand. The PT has to be based on an AC VA spec. For cap-input rectifier, the AC VA is about 1.4 times the DC Watts, so the PT is already 319VA, say 340VA with driver and preamp stuff. Heaters are 6.3V at 3.6A to 6.4VA, plus maybe 1.5A for little tubes, 6V at 8A or 50VA. So this is a 400VA core.

[Ritchie200]

IMHO, 120 tested Watts from four 6L6GC in stage-amp duty is risky. I'd use four good EL34, six 6L6, or two 6550.

I would spread "good" across to the 6L6 and 6550's.  Also, did you mean to say four 6550's?  Maybe two very good old Kt-88's or KT-90's.  I'm not sure any of the new tubes would last very long pushed this hard.  Loud and clean?  How about 4 good KT-88's in an UL design?  They could go all day at 120w or more.

Explain the cost of tube replacement and the fact that he will not notice a difference between 100 watts and 120 and he may come to his senses.  Or if you want to stay with 6L6 (like PRR said), add tubes and design for reliability so you don't get a rep for builds that go poof.  If this is for his stage rig - it's the last place he needs a failure...

Jim

[rzenc]

IMHO, 120 tested Watts from four 6L6GC in stage-amp duty is risky. I'd use four good EL34, six 6L6, or two 6550.

I did not say I want 120W out of 4x6L6GC, it was just a 'number figure to proper size O.T.'
But I believe a bigger O.T. sounds better with big amps.

This is a conversion, so we will use reminiscent P.T. and choke and change O.T.
P.T. puts out  ~460VDC @ reservoir cap. The only data I found was with plates @ 450VDC. It's 2% of difference. At full roar it kept ~430VDC.
The reason behind O.T. change is because it craps out on bass notes.

You hope for 120W out, you need 228 Watts of DC. At 450V-500V this is 506mA-456mA DC demand

So I need to ask for primary current handling ~550mA'sD.C.? hmmm it seems that I would be best upgrading iron even further then I thought... I don't want it to color bass notes... now the real McCoy: Would a 150W O.T. (F.R. 30Hz-33KHz) handle it?? It's imperative that bass notes comes out clean and clear.

Thanks in Advance.
Best Regards,
Rzenc

[rzenc]

IMHO, 120 tested Watts from four 6L6GC in stage-amp duty is risky. I'd use four good EL34, six 6L6, or two 6550.

I would spread "good" across to the 6L6 and 6550's.  Also, did you mean to say four 6550's?  Maybe two very good old Kt-88's or KT-90's.  I'm not sure any of the new tubes would last very long pushed this hard.  Loud and clean?  How about 4 good KT-88's in an UL design?  They could go all day at 120w or more.

Explain the cost of tube replacement and the fact that he will not notice a difference between 100 watts and 120 and he may come to his senses.  Or if you want to stay with 6L6 (like PRR said), add tubes and design for reliability so you don't get a rep for builds that go poof.  If this is for his stage rig - it's the last place he needs a failure...

Jim

I a real fan of 6550's...But chassis is too compact to install them. I'm pretty sure he would be best served by katties88/6550 then 6L6 but he likes them...Also, I did not promise any watts figure, just said it could be around 80W... the more the better, but he really doesn't need to know it... afterall a so-called 80W could be paired with others 80W and kick their asses

Or if you want to stay with 6L6 (like PRR said), add tubes and design for reliability so you don't get a rep for builds that go poof.  If this is for his stage rig - it's the last place he needs a failure...

This is the bottom line behind conversion..it was a Laney GH100L, said was because now it's a RZC 80 Reaper shoppe    :evil4:
He is gigging with the same circuit I'm going to install inside the chassis but a pair of 6550's and the other player in the band happens to have another GH100L and my amp is kicking it's butt night after night...tonewise he says all sound guys comes to see what the hell is that red stuff on the stage. It's about 4 - 5 months with my amp on the road and no complaints at all. Of course I feel proud of it, please don't get me wrong, we have discussed how today mass market amps sucks bad...

I would love to put 4xKT88 in U.L. but chassis space is very limited. Since we already have a drilled chassis and a tolexed cabinet we didn't bother to make new things, so that we can keep costs down...

We installed his O.T. onmy amp and bassnotes were as crappy sounding as was his laney, so he asked me to change it in order to get the same kind of strengh on lo end.

Thanks for your kind attention Jim.
Best Regards,
Rzenc

[Ritchie200]

Hey Rzenc - will you be at Interlagos this weekend?!?! Go Massa/Senna/Rubinho/diGrassi!

I'm sorry, maybe I misunderstood.  You mention that the OT is yucking things up and you have one to replace it.  And you are still talking about a conversion and pushing the 6L6's harder than they are now?  What is in the amp now, 6L6 or EL34's?  A OT rated at 150 watts for that freq spread would handle anything 4 6L6's could throw at it at geetar levels.  Sounds like this is a pretty simple conversion if you are limited to what you had before, only changing the OT and the board - if I am understanding you correctly!  THese were loud amps to begin with.

Congrats on your RZC 80 REAPER! :evil4:  

Jim

[PRR]

> did you mean to say four 6550's?

Yes; thanks for the catch.

[PRR]

> O.T. change is because it craps out on bass notes.

The OT or the _speaker_?

If he is going to play the bottom octave on Bass, he needs a BIG speaker to make big bass.

Put a 7 ohm 100W resistor (fourteen 100r 10W parallel) and a 1 ohm 10W resistor in series across the amp. Hang the speaker across the 1 ohm. If that plays well (though very weak), then the amp has the horsepower and the speaker can't take it.

Put more iron in the OT. If you are inclined to copy the existing OT, tell the winder to use the same size laminations but more of them, stack height 1.5 to 2 times the original.

2X stack will increase the winding resistance somewhat (longer length of turn). That's probably not a problem here.

A smart winder will take this idea and translate to a larger lamination and more-square stack, which can get the winding resistance back down (both shorter turn and fewer turns). The audio difference is small; the physical dimensions may be the deciding factor (do you have width, etc?).

A 60Hz PT is 20% smaller than a 50Hz PT. A 40Hz transformer must be larger than a 50Hz PT. Larger yet, because primary-current distortion is not a problem for 100W PTs (the power company absorbs the error) but distortion is a big problem for clean output from a limited "generator" (tubes).

Put more tubes in there! The bass craps-out because the flux level gets "near" saturation and inductive current rises. More tubes, more available current. You can drive a transformer VERY low if you do not mind putting 10 times more energy into useless reactance than useful output power. Design your audio load for say 2K, then put in enough parallel tubes to drive 200 ohms. With tubes, that gets stupid fast; but don't skimp.

[PRR]

> ~460VDC .... At full roar it kept ~430VDC.

That's not your problem. The PT is fine.

[PRR]

Five or so years ago I got a new Mesa/Boogie Dual Rectifier Trem-o-verb with the matching huge and heavy 4x12 Rectifier cab (size-wise it makes the Laney 4x12 look like toy). One day I decided to experiment and plugged the Laney into the Mesa cab. Wow, what a difference! All those years it wasn't the Laney GH100L that was "average", it was that the Laney cab wasn't doing the amp justice!

http://reviews.harmony-central.com/reviews/Guitar+Amp/product/Laney/GH100L/10/1

[rzenc]

Hey Rzenc - will you be at Interlagos this weekend?!?! Go Massa/Senna/Rubinho/diGrassi!
I'm sorry, maybe I misunderstood.  You mention that the OT is yucking things up and you have one to replace it.  And you are still talking about a conversion and pushing the 6L6's harder than they are now?  What is in the amp now, 6L6 or EL34's?  A OT rated at 150 watts for that freq spread would handle anything 4 6L6's could throw at it at geetar levels.  Sounds like this is a pretty simple conversion if you are limited to what you had before, only changing the OT and the board - if I am understanding you correctly!  THese were loud amps to begin with.
Congrats on your RZC 80 REAPER! :evil4: 
Jim

Unfortunately not... Too much work to do.... you know what they say here? when you work too much, you don't have time to earn money...

What I said is that we took the Laney O.T. and installed on my amp. Bass notes which were clean and clear became muddy, saggy, with none of that beautiful round sound...
Laney clains 100W for this amp, ok... i'm not saying that it can't deliver this figure either, I'm saying we did not like how it sounds on bottom octave.
We played both amps on different cabinets, to name a few:
Peavey w/ Celestion G12-65;
Carvin w/ Celestion Vintage30's;
Laney w/ H.H.;
Marshall w/ Celestion G12-30H;
MesaBoogie - not sure what was inside;
Custom made Cab w/ Eminence Cannabis Hex;
Tech 21 w/ Celestion Greenbacks;
Marshall 1960 with EV12L
All them are 4x12". Of course each has it's own sound/characteristics, but what remained one after the other was how different both sounded on bass notes...So he insists on changing the O.T.


did you mean to say four 6550's?
Yes; thanks for the catch.
Put more tubes in there! The bass craps-out because the flux level gets "near" saturation and inductive current rises. More tubes, more available current. You can drive a transformer VERY low if you do not mind putting 10 times more energy into useless reactance than useful output power. Design your audio load for say 2K, then put in enough parallel tubes to drive 200 ohms. With tubes, that gets stupid fast; but don't skimp.

I would really like to do so, but chassis has a very limited space, tube sockets are pretty close together. I did try to put 4x6550 and the average distance between tube glass container is 5mm - less than 1/4" - it would not provide enough air circulation around the tube. I thought about a small P.C. fan but I don't know if it will raise noise problems...

> ~460VDC .... At full roar it kept ~430VDC.

That's not your problem. The PT is fine.

At least we can keep something besides chassis and knobs  :knob:

Five or so years ago I got a new Mesa/Boogie Dual Rectifier Trem-o-verb with the matching huge and heavy 4x12 Rectifier cab (size-wise it makes the Laney 4x12 look like toy). One day I decided to experiment and plugged the Laney into the Mesa cab. Wow, what a difference! All those years it wasn't the Laney GH100L that was "average", it was that the Laney cab wasn't doing the amp justice!

http://reviews.harmony-central.com/reviews/Guitar+Amp/product/Laney/GH100L/10/1

Indeed, it really looks like a toy, but this is not his cab. Actually he uses the Peavey cab loaded w/ Celestion G12-65. His cab is wider and deeper than Laney. Sounds very nice with my amp.

Ok, so far so good. I will shoot for 2K8 plate to plate.
I will measure O.T. size and post back soon.

Thanks for your kind attention.
Best Regards

Rzenc