HRM needs some Lovin'

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martin manning
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Re: HRM needs some Lovin'

Post by martin manning »

The best arrangement for AC power is hot (black) to fuse holder center lug, fuse holder side lug to AC power switch (1), AC power switch (2) to transformer primary (1), transformer primary (2) to AC neutral (white). The safety ground (green) should go to a dedicated chassis ground (no other connections at this point), and the green lead should be left long so it will be the last wire to become disconnected if the AC power cable is pulled out. You can certainly test the PT and rectifier wiring once you have it wired up, in fact I like the test-as-you-go system. 6A is robust for the heaters, and will support EL34's if you want. The only down side is the filament voltage may be a bit high with 6L6's.
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norburybrook
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Re: HRM needs some Lovin'

Post by norburybrook »

You should have your AC ground as a separate ground only. That's the legal/proper way of doing it. I think it should be longer than the live/neutral leads as well so in case of the cable being pulled out the earth will always stay connected.



Marcus
fred.violleau
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HRM needs some Lovin'

Post by fred.violleau »

Thanks Martin! I will then correct my wiring accordingly.

When you say filament voltage may be a bit high with 6L6s, would impact the tubes life? The sound (I am assuming not, but not sure about it)? Or both?

Humble and Dumble,

Fred.
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martin manning
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Re: HRM needs some Lovin'

Post by martin manning »

I should have reviewed... you are building a 50W with four preamp tubes, so your heater demand is only 3A with 6L6's, 4.2A with EL34's. According to the tube manufacturers, +/- 10% on heater voltage is acceptable. See what you have for filament voltage when all the filaments are wired up. If you are under 6.9VAC you're ok. You may have reduced tube life if you are on the high side of +10%, and the resulting increase in cathode temperature will increase the anode current a bit.
fred.violleau
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Re: HRM needs some Lovin'

Post by fred.violleau »

norburybrook wrote:You should have your AC ground as a separate ground only. That's the legal/proper way of doing it. I think it should be longer than the live/neutral leads as well so in case of the cable being pulled out the earth will always stay connected.



Marcus
Hey Marcus, thanks for jumping in, the more the merrier ;)

I saw on one of your post the avantages of having a dual biasing system. Would I benefit from it if I wanted to be able to swap power tubes (6L6 for El34)?

Cheers and "almost" happy new year !

Fred, learning everyday!


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norburybrook
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Re: HRM needs some Lovin'

Post by norburybrook »

Fred,

martin and I were typing at the same time it would seem :D Martin has been my mentor in my amp building apprenticeship so I would always bow to what he says :D

I've never actually tried a dial bias setup.

the main difference between EL34's and 6L6 is the current draw, so if you have a transformer for the higher current draw EL34's the you can always swap to 6L6 with a bias change. You can get into trouble the other way round though if your transformer can't handle the extra current draw of EL34's

I would add that there's bugger all difference in sound on an ODS between 6L6 and EL34's :D others may comment otherwise but I'd say it's very subtle.



Happy new year to you too,



Marcus
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martin manning
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Re: HRM needs some Lovin'

Post by martin manning »

The D power amp is a clean sort of thing, so I agree the tube type shouldn't make too much difference. And, I think it's safe to say you are now one of the more experienced people on this forum now, Marcus. We are all still learning, though... or at least we should be!

Separate bias pots can be useful to match up the idle current, both initially and as the power tubes age, and that applies equally for 6L6 and EL34. The down-side is you need to find room for another pot, which can also be wired as a balance if you like.
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norburybrook
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Re: HRM needs some Lovin'

Post by norburybrook »

martin manning wrote: Sun Dec 31, 2017 10:35 pm The D power amp is a clean sort of thing, so I agree the tube type shouldn't make too much difference. And, I think it's safe to say you are now one of the more experienced people on this forum now, Marcus. We are all still learning, though... or at least we should be!

Separate bias pots can be useful to match up the idle current, both initially and as the power tubes age, and that applies equally for 6L6 and EL34. The down-side is you need to find room for another pot, which can also be wired as a balance if you like.
Thanks Martin, i agree we should all be constantly learning, I plan to do that until the day i shuffle off this mortal coil. I like the fact that no matter what you do in life you can always be better and so therefore learning is a constant companion. I would also add it's good fro you in that it keeps the old grey matter active. Couple that with an active body and hopefully age will not be a downward spiral.

have I veered off topic? It's new year and I've had a bottle of wine :D


happy new year all.


Marcus
fred.violleau
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Re: HRM needs some Lovin'

Post by fred.violleau »

Glad to share all that with you guys!cheers and all the best for 2018,starting with a good health and the best sound that is around!

Happy new year y’all!


Humble and Dumble,

Fred.
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Re: HRM needs some Lovin'

Post by fred.violleau »

Another quick one :
The center taps of the Power Transformer should all be grounded on the same star ground ? Should the three center taps be grounded. I noticed that on the previous amp, the center tap of the high voltage was not grounded hence my question.

Thanks!


Humble and Dumble,

Fred.
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norburybrook
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Re: HRM needs some Lovin'

Post by norburybrook »

On Dumbles the main HT center tap is grounded with the ground from the bias rectifier board and the main power filter caps. Nowadays some people put an HT fuse on there (500ma).


If there's no center tap on the HT it's because it's running a full wave bridge rectifier to get the required voltage. It's dependent on the trasformer.

marcus
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martin manning
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Re: HRM needs some Lovin'

Post by martin manning »

Fred, you haven three center taps: one for the high voltage secondary, and one for each of the filament windings. Ground the Red-Yellow (HV) and the Green-Yellow (6.3V) at the First filter ground. The Black-Yellow is on the 5V winding (Yellow leads). If you are using that for relay power, I'm assuming you will have a voltage doubler on that winding, so don't ground the CT, just heat-shrink it and tie it out of the way.
fred.violleau
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Re: HRM needs some Lovin'

Post by fred.violleau »

Again thanks for your answers! Martin you are right and I made a mistake on my observations. It was the centre tap for the relay that was not grounded, not the high voltage one.

I will power up as all seems fine, and will report my voltages here in a bit!


Humble and Dumble,

Fred.
fred.violleau
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HRM needs some Lovin'

Post by fred.violleau »

2.8v on every heater, 383v on each red high voltage, 0v on the 2 yellow. I will try to put in the tube and make measurements.


Humble and Dumble,

Fred.
fred.violleau
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Re: HRM needs some Lovin'

Post by fred.violleau »

All tube light up. I might move on to the next stage!


Humble and Dumble,

Fred.
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