The 6550 Experience

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niknik
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Re: The 6550 Experience

Post by niknik » Wed Jan 31, 2018 6:29 pm

Is this a correct RFC, for major project?

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Re: The 6550 Experience

Post by revolver1 » Thu Feb 01, 2018 12:57 pm

By my own admission I'm not the most technically knowledgeable guy on the forum so i need a bit of advice.

I will be using a 2x12 loaded with a V30 and a Scumback H55 so I have concerns about the power handling. Also that a 100w WCM Superlead may overwhealm another amp run in parallel via an ABY switcher.

Looking at the price of NOS Tungsol 6550's and considering the future cost of re-glazing my current amp empire I'm wondering could this West Coast mod be applied to a 50watt?

The standard O.T is 3.7K primary so not far off the preferred 4K. Would that actually work?

I'm wondering if I got a P.T custom wound what B+ would I be shooting for to do the high voltage thing in a 50watt format?

I was thinking of building it in a 100watt chassis so I could go with higher filtering 100w style or even consider a separate dual tap on the P.T for a separate screen supply and I'd have the extra holes for the added post choke filter cap.

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Re: The 6550 Experience

Post by daveweyer » Thu Feb 01, 2018 5:30 pm

That RFC is 5" long! You won't need anything that big, just a few mh. You can even make it yourself by winding about 50 turns of #26 magnet wire around a 200 ohm 3 watt resistor. Or, there are still plenty of suppliers for small value chokes.

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Re: The 6550 Experience

Post by daveweyer » Thu Feb 01, 2018 5:34 pm

The West Coast mod works fine on 50 watt amps, we did a lot of them. 3.7K is fine for 6550s or KT120s, and you won't need a lot of plate voltage, probably around 450 volts DC. Mercury, Triode, and others have lots of those available, 200 or 250 ma will do.

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Re: The 6550 Experience

Post by revolver1 » Fri Feb 02, 2018 1:33 am

Thanks Dave, great news!

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Re: The 6550 Experience

Post by daveweyer » Fri Feb 02, 2018 1:02 pm

Here is a power tranny that will cover a lot of situations; Triode Electronics Universal power 40-18069. This will make a nice 50 watt, or up to probably 150 watt, perhaps more since it has 500ma of secondary current. Extra windings can give you 586 volts of B+, and, there is a separate bias winding.

For outputs in the guitar world, a rule of thumb is that a 100 watt hi-fi type output transformer will be a 200 watt guitar OT. This is because you don't need a 20 HZ response with a guitar, or even a 30HZ for that matter. So for instance, the Dynaco A 431 makes a wonderful 120 watt guitar OT. I have some old amp chassis here which use that very transformer for over 100 watts output.

If you want to make a Major, you only need a 100 watt hi-fi type transformer.

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Re: The 6550 Experience

Post by niknik » Fri Feb 02, 2018 3:55 pm

daveweyer wrote:
Fri Feb 02, 2018 1:02 pm
For outputs in the guitar world, a rule of thumb is that a 100 watt hi-fi type output transformer will be a 200 watt guitar OT. This is because you don't need a 20 HZ response with a guitar, or even a 30HZ for that matter. So for instance, the Dynaco A 431 makes a wonderful 120 watt guitar OT. I have some old amp chassis here which use that very transformer for over 100 watts output.

If you want to make a Major, you only need a 100 watt hi-fi type transformer.

It's good to know, Dave! :D
In my mini Major built I have a Dynaco A-431, so it is able to handle 4 kt120, it's right? Primary impedence is 4300 ohms, do I need to do a mismatch between output impedence and cabinet?
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Re: The 6550 Experience

Post by JimiJames » Fri Feb 02, 2018 6:20 pm

Are there brands of 6550's that can handle the higher voltages better than others (even though the same type tube)?
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Re: The 6550 Experience

Post by neikeel » Sat Feb 03, 2018 10:13 am

JimiJames wrote:
Fri Feb 02, 2018 6:20 pm
Are there brands of 6550's that can handle the higher voltages better than others (even though the same type tube)?
Thanks :-)
Best I have settled on is the Svetlana Winged Cs (running them in a modded 71 SB and a warmed up Orange clone) but I am sure there are lots of other good ones too.
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Re: The 6550 Experience

Post by daveweyer » Sat Feb 03, 2018 3:10 pm

Four KT 120s might be a bit of overkill for a 100 watt amp, KT 90s would be more than sufficient. Good 6550s as well.
Two 6550s with 600 plate and 300 screen volts produce 100 watts.
If you like four tubes, four 6550s with 600 plate and 300 screen volts will produce 120 watts from a Dynaco A 431. The reason the power doesn't double is because the P-P plate load is still about 4K, but you get a little more power because of the added cathode current capability--also less distortion at rated power. This circuit is very easy on power tubes; that's why I used to use it for Jimi and others at West Coast Organ and Amp back in the day.
Any decent OT with a 4K or even 5K primary will work with 4 tubes. What I often did was increase the plate voltage to take advantage of the higher primary impedance; I made a series of amps using the Stancor A8053 hi-fi transformer, an absolutely marvelous sounding 50 watt transformer with a 5K primary which would heroically output up to 175 watts from four 6550s. I would use 725 volts on the plates and 400 on the screens.
Another rule of thumb is that more plate voltage requires higher primary impedance in the OT, swing more voltage at lower current for the same power output.

The newer Tung Sol tubes from Mike Matthews recently made in Russia have been rather amazing in their cathode current and plate sat current ratings, from what I can see, a match for old American tubes from the '70s.

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Re: The 6550 Experience

Post by niknik » Mon Feb 05, 2018 5:51 pm

Thanks Dave, I will try four 6550s in my major with the dynaco A- 431 (now I'm using two kt88s).
I have to disconnect UL, and go with separate screen supply arrangement. Currently I'm using a triode p-782 pt, which give me about 560v b+
Maybe for now I can go with this pt, but I have to calculate how many ampere the filaments of four 6550s plus six preamp tubes require, like in your project, 'cause this pt has only 5A for 6.3v secondary. :scratch:
About rfc, I found some that are looking like those in the pics of your amp, are 24uh and Freq.Mc 25-40 ok?
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Re: The 6550 Experience

Post by daveweyer » Mon Feb 05, 2018 6:35 pm

Give about 1.6 amps each for the 6550s, and then about 3 amps for the other tubes. That said, you can obtain more than the rated output from the filament windings on most transformers, depending on the wire size they used and the amount of core available. Say you have a PT rated at 500ma on the HV winding, but you will only use 350ma of that during any given period, this leaves extra dissipation available in the PT which can be used by heating up the filament winding a bit more. Plus you have extra magnetic capacity available, so the PT won't be overloaded magnetically by the extra current in the filament coil. All trannies are different, but most I've seen will easily allow 20% more filament current. To be honest, I have actually doubled the filament current and gotten away with it, but it does make the transformer run hotter--when the filament voltage starts to drop noticeably, that's probably enough overload.

And Tek, I just found a stash of 2N591s and 2N725s, probably enough to construct three full fuzz units and part of another. I also found three 2 meg pots which you can use in the AC feedback path. I have lots of 1.8uf caps, they are 1.5uf caps which all measure higher.

I'll let you know when I get these devices measured on my Tektronics beta meter, and then, if they fall in the right current gain range, we'll figure out a way to get them to you.

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Re: The 6550 Experience

Post by niknik » Wed Feb 07, 2018 10:38 am

Dave, I'm really into this your Major project, it seems really awesome! This weekend I will start to do some of the mods. I just ordered a 6kz8 so I have got all the preamp tubes required.
For now I will go with A-431 and p-782 transformers, so I presume I can go with lower than 500uf main filter capacitors.
For screen supply, can I use a 10H 250mA choke, in place of the 645ohm 25w res? It misures 285 ohm.
In the lower right of the schematic, is it a 100m res, in parallel with .5uf capacitor to ground?
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Re: The 6550 Experience

Post by Roe » Wed Feb 07, 2018 11:01 am

daveweyer wrote:
Fri Feb 02, 2018 1:02 pm
Here is a power tranny that will cover a lot of situations; Triode Electronics Universal power 40-18069. This will make a nice 50 watt, or up to probably 150 watt, perhaps more since it has 500ma of secondary current. Extra windings can give you 586 volts of B+, and, there is a separate bias winding.
yes, this classictone PT, sold by triode, seems perfect. I'm also considering the plexi style 2k2 100w OT made by classictone. Do you think it may work for a westcoast amp if you avoid voltages above 530v?
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Re: The 6550 Experience

Post by daveweyer » Wed Feb 07, 2018 1:57 pm

Niknik, you will need the resistor if you are planning to use the mosfet regulator, simply because the choke will not lower the voltage applied to the drain of the mosfet. The purpose of the 645 ohm resistor is to drop the voltage to the mosfet regulator when the outputs are drawing a lot of current, and this means in turn, that most of the power dissipation will occur in the resistor, and not the mosfet. In other words, it is better to get the resistor hot than the mosfet, because there is only a small aluminum heatsink on the mosfet which is thermally connected to the chassis for heat dissipation. No one knows how much heat the chassis can absorb. So find a 25 watt resistor, 650 ohms will do, or you can parallel two or more 10 watt resistors to get the value needed.
The 100meg resistor is to drain off the voltage from cap on the gate of the mosfet should all other means fail. The gate impedance of the mosfet is so high that the charge will remain on that cap for weeks; the idea being that when the amp is fired up, everything starts from zero and gradually charges, the time constant on a 1 meg and a 1uf cap is 1 second, that being the time it takes to charge the gate of the device to 60% of the voltage.

And, when you get your mosfet, the first thing to do is attach the small 12 volt zener from the source to the gate, keeping the pins shorted while you do this. Once again, the gate impedance is so high, that a little static from your hands can blow the junction. The Zener also keeps the gate from ever rising more than 12 volts above the source during operation.

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