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Thread: SE amp with 6550 tube

  1. #1
    Alf
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    SE amp with 6550 tube

    Hi, I am looking for a good design for a single ended guitar amp schematic using a 6550 tube ( because I have 16 Svetlana's lying about which I like to put to use).

    So far I haven't designed any amps myself and I could do with some help.

    I have a few questions first , maybe someone could give me some advice ?

    What PT do I need ( secondary voltages) 275 V or 310 ? I'm going to use a bridge rectifier (4007's)
    Then I want to use a choke for the 6550 as well as the preamp tube(s), to get rid of as much hum as I can.

    I was thinking of using a 4H 225 mA Hammond choke , should do the job I think.

    And I've also got a Hammond 125 ESE OT and would like to try this one first or do I need something heftier ?

    The Duncan PSU II designer is a great help.

    Thanks beforehand,

    Alf

  2. #2
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    that choke should be perfect.
    the 310vac tranny would be better for a 6550 imo. 6550's can run at high voltages and 400-430v would seem about right for a SE design.
    not sure about the OT, but if its rated for 10w or more it would seem about right to me. hammond tends to rate things conservatively as they are designed for hifi use.

  3. #3
    Alf
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    I found a schematic from Pierre Debbs which I saved some years ago.
    Hope he doesn't mind my posting it here !

    I think it will take a 6550 as well with some changes.
    Anybody who would like to comment on it ?


    http://i291.photobucket.com/albums/l...chemKt88SE.jpg

    He uses a bigger OT , I will first try the 125 ESE and see what that gives.

    Thanks for the answer, Black Labb !

    Alf

  4. #4
    Alf
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    I ordered the Xformers and have to wait for a while.

    Hasn't any of you built a real SE amp . Only one response seems a bit disappointing.

    Sorry to be so insistent but I could do with a bit more experience from folks on this forum.

    Alf

  5. #5
    Old Timer
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    What do you need to know? You seem to have a workable schem, your 125ESE would seem fine...

    If you really want to minimise hum in a SE design you might want to consider dc biasing the 6.3VAC heater supply. You want 35-40vdc, you might be able to achieve this by tapping the dc voltage generated at the 6550 cathode resistor depending on final voltages, or tap the dc voltage from the first filter cap via a 220K 2W+ resistor going to one leg of the heaters, then a 22-27K 1/2W reasistor from the same leg of the heaters to ground...adjust value of the 22K-27K to get your 35-40vdc. There should be no ground reference, or centre tap in the heater circuit. Heater circuit should be a twisted pair, not a single wire daisy chain as shown in many old SE schematics.

    Observe the Fender 5F1 layout (noting heater issue above), combined with the grounding scheme shown at Hoffmanamps/EL34 world.

    Perhaps go to 1K 5W for the screen grid resistor feeding pin 4 of the 6550...other than that, if you have any specific queries ask away.

    If you just want random feedback/suggestions, well...be careful what you wish for, ask 100 guys and you'll get at least 101 differing responses.

  6. #6
    Alf
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    This is the kind of information I need MWJB !

    Especially the screenresistor is what I would like to know.

    How much does the screenvoltage have to be lower than the platevoltage ?

    Of course it depends on the platevoltage I will get from the PT.

    I am going for the Hammond 370 HX , PT. I have to see what it will deliver after a fullwave rectifier.

    The suggestion of raising the filaments to 40 volts above ground is a good idea which I read about but have never tried so far.

    My goal is to get the amp to be as quiet as possible. That's why the platevoltage will be taken off after the choke. ( it can handle 225 mA) .

    First I will try to feed the filaments with ac and if ther้'s hum I will go for dc.

    What I really would like to see is some other schematics , that is for guitar use.


    Thanks, Alf

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    Supporting Member tubeswell's Avatar
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    Quote Originally Posted by Alf View Post
    What I really would like to see is some other schematics , that is for guitar use.
    Hi Alf

    Check out AX84.com (you have to sign-up as a member to see most stuff)

    AX84.com - The Cooperative Tube Guitar Amp Project

    and also Angela Amps has a couple (including a parallel SE amp).

    Angela Super Single-Ended 6V6 Guitar Amp Project


    You can also find some ideas at the Amp Garage

    The Amp Garage :: View topic - access to high powered single-ended transformers - thoughts?
    Building a better world (one tube amp at a time)

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    "How much does the screenvoltage have to be lower than the platevoltage ?" There is no specific voltage margin by which the screens should be lower than the plate...it is good practice to have them lower than the plate, beyond that it is a case of personal preference. SF champs for example often run their screens higher than the plate. With EL34 & 6550 the 1K is used to give the tube an easier life...it's proven & it works, so go with it. If you were looking to significantly alter the screen voltage then another power supply dropping node & filter cap would be the way to go, rather than ever larger screen grid resistors. However, this shouldn't be 'necessary' in your design & using the brute force choke filter usually means that plates & screen will be close in voltage.

    Ooops, on checking, I see your design already has a dedicated screen supply dropping node.

  9. #9
    Alf
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    MWJB ,


    In the design I want to use the screenresistor is 220 ohms .

    I personally think that going a bit higher to e.g. 1K would be on the safe side.

    How much does it affect your sound and in what way , I mean what does it do for harmonics ?

    Tubeswell , I know the ax84 site , thanks for mentioning it again.
    I don't visit it regularly though, maybe I should.

    And the ampgarage I mainly visit for the Wreck and Humble amps !

    Didn't know they had SE amp designs.
    Will have a look there too.

    Alf

  10. #10
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    Well there's already a dropping resistor in the power supply for the screens, so 220R might be fine (if you need to buy a resistor I'd still go with 1K, then that frees you up to experiment with the screen supply dropping resistor in th B+ rail. The screen won't draw much current so the difference between 220/470/1000R isn't going to be huge, a few volts...maybe too subtle to hear?

    Be aware that aggressive voltage dropping at the screen supply node in the B+ rail will also have a knock on effect at the preamp stage, build as is (but with a dedicated 1K 5W at pin 4) and evaluate by ear.

  11. #11
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    For a first time build your schematic should work well driving a 6550. If you stay close to the Champ schematic, you can build just about any SE amp, that is gain > EQ > gain > output. I started with the Champ, but used a Gibson EQ and a 6L6 output at 420V B+. Just by changing the cathode resistor, I've also used EL34, 8417, 6L6 and 6550, all in triode mode. If your PT and OT are big enough, the sky is the limit. If you have a lot of 6550's, maybe try parallel oputputs, or a 5.1 Guitar Theatre...
    Black sheep, black sheep, you got some wool?
    Ya, I do man. My back is full.

  12. #12
    Alf
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    It's not my first time build . it will be my fourth from scratch that is but it's going to be my first SE amp.

    As you may have read I'm first going to try and use the Hammond 125ESE OT which can deliver 15 watts and if that doesn't work as I expect I will buy a bigger OT.

    I just happened to have this schematic and are going to give it a try because it's a one knob amp. I will tune it tonally without a regular tonestack.

    If you use all different output tubes you will have mismatches with the same OT won't you ?

    Have you got suggestions for a possible tonestack ; what Gibson eq did you use ?

    Alf

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    HTH
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    I personally think you're pushing it using a 125ESE rated for 70mA - that 6550/KT88 will be pulling 100mA.

    I know guys have used this exact same setup and reported that the OT was fine, I'd personally play it safe and go with the 125GSE (I used one in an EL34 build that has the option to use KT88s and it sounded monstrous).

    Talking of SE amps, I've been in love with them since building my SE EL34 amp. I've been planning on a SE amp in the 40w range and was considering four 6L6s, but a pair of parallel 6550s would give around 35w and be more compact. The trigger for this project is the 400mA Hammond choke I got cheap - its as big as a 50w Marshall OT, ha ha.
    HTH - Heavier Than Hell

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    Alf
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    Thanks for the advice HTH . What OT did you use for your SE EL34 amp ?

    Recently I've seen a schematic with the Hammond 125ESE being used with this tube .

    Alf

  15. #15
    HTH
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    Quote Originally Posted by Alf View Post
    Thanks for the advice HTH . What OT did you use for your SE EL34 amp ?

    Recently I've seen a schematic with the Hammond 125ESE being used with this tube .

    Alf
    I used the 125GSE which is good up to 100mA, while the 125ESE is only spec'd up to 70mA.

    In my EL34 SE build, I had around 172v on the EL34 plate with a 330R cathode resistor giving around 69mA across the EL34. With 6CA7s and 6L6s it was more like 75mA, but the 125ESE OT would probably be fine with such a small amount of extra current going through it.

    However, I just feel more comfortable overspecing things and it also means that the amp can be more versatile and run 6550s without any trouble. The PT was WAY overspec'd (200mA on the HT), as it cost the same as the lower spec'd ones and I again figured that it'd give more options. Surprisingly, the heater voltage didn't run high with such underloading and I was getting bang on heater voltage at 6.4v
    HTH - Heavier Than Hell

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    125ESE is rated at 80mA, not 70mA.

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    Alf
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    But not good enough for a 6550 if it draws 100 mA ?

    Alf

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    Why would you need to draw 100mA (360vdc on the plate)? You're going to want >400vdc-ish on the plate, 6550 is a 40W tube, biased at 75-90% = 90mA max. There are SE 6550 guitar amps out there that run less plate current than 70mA and still sound great...I'd see how it sounds from 60-80mA, and a couple of points in between.

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