Plate Voltages?

[Bassist001]

so, im new to the whole amp building think and i was just curious about plate voltages. Because some amps you see have really high voltages and some not so high. so what would the difference be between high voltages and the lower side of things? Thanks

[MWJB]

High plate voltages bring better power, cleans, better dynamics & low end.

Lower plate voltage can be "browner", warmer, ultimately less output power.

Most guitar amps are designed for a particular tone, maximum potential power might not always sound as pleasing on the ear.

A word on voltages listed on 50's & 60's Fender schems, the are all wrong, 10-15% low.

[Bassist001]

thats exactly what i was looking for! thanks! so in speaking about power transformers. Would that be where you would hear the biggest differences? In the amount of voltage your gonna be sending out?

[MWJB]

Yes the power transformer is the critical component, then the type of rectifier - a solid state rectifier takes the AC from the PT and bumps it up by a factor of ~1.4, a GZ34/5AR4 by a factor of ~1.3, a 5V4 by ~1.26, a 5U4 by ~1.2, a 5Y3 by ~1.1 (all assuming a moderate current draw, hot biased cathode-biased amps may pull the final B+ down due to a higher current load).

E.g. a centre tapped PT may be rated at 350-0-350VAC at 150mA (or better), this will equate to a solid state rectified voltage of around 480-490 vdc, on a pair of 6L6/EL34 idling at 30-35mA per tube, plus a few 12A#7 tubes. Sub the recto for a GZ34 and you'll see 475-485vdc. As long as your bias pot has reasonable sweep there is nothing to stop you aiming for best power voltage with a SS recto, & subbing for a GZ34, or 5U4 to brown out the sound as desired (5V4 & 5Y3 may struggle in the above example). You will need a 5V 3A rectifier winding on the PT to do this. It's not an excact science because of manufacturing tolerances, bias conditions, wall AC in your area & time of day, whether the PT vendor/mfr is full of shit or not, whether they quote "loaded" (like I do) or "unloaded" voltages. As long as you get to within +/- 10vdc I wouldn't throw your toys out of the buggy.

You also need to consider heater current, 6L6 draw 0.9A each, EL34 draw 1.2A each, 12A#7 tubes draw 0.3A each. Total amps drawn should be siginificantly less that the PT's rating, allow +50% min, 100% overrating is good but you might need to get your PT custom made.

Some PTs are not centre tapped at the B+ winding, these must be bridge rectified with a solid state, 1.4 times the whole of the winding (expressed as 0-380).

As long as you are not building something too outlandish, somebody probably already makes your PT.

[Bassist001]

Wow! That was a wonderful explanation!! Thank you! It makes so much more sense now! But, while we're on the subject I just have.two more questions.1. What exactly causes.the jump up in voltage through a rectifier? Is it just simply the transition from ac to dc? Or does say a tube rec actually amplify it? And if that how would a ss rec amplify it? Ha ha and 2. What exactly is the 5v 3amp winding doing? Thanks

[MWJB]

1) It's not entirely the tube rectifier, the filter cap after the rectifier plays a part too, if you just run a rectifier without the filter cap you will get about the same in dc as the PT puts out in AC. Rectifiers have internal resistance, so those with a higher internal resistance (5Y3/5U4) don't allow so much voltage to develop as those with a lower internal resistance (5V4/5AR4). E.g. a 5Y3 is rated at a 50v drop at its maximum current capacity, a 5AR4 is rated at a 10v drop at it's maximum current capacity. A SS rectifier has no internal resistance (unless you add some, like with a Weber Copper Cap), hence it allows maximum potential voltage (as long as the other factors, like rated current draw, support this. Google "tube rectifier comparison", or similar, you are bound to hit on a chart displaying their characteristics.

2) The 5V 3A winding is the filament winding for the tube rectifier, if you don't have this then you have to use a solid state rectifier, or add a 5V 3A auxilliary transformer (which in reality you are unlikely to do). Amps that draw a lot of current (4x6L6 or EL34, 6550) tend to be SS rectified as they often demand more than a tube can deliver. Some manufacturers have built amps with multiple tube rectifiers in parallel to handle higher current loads, but this makes for a very expensive PT.

[Steve Conner]

1. What exactly causes.the jump up in voltage through a rectifier? Is it just simply the transition from ac to dc?

Yes. Well really it's an accounting thing. The AC voltage is a sine wave, and the value quoted by the PT maker, and measured by your meter, is RMS.

The instantaneous voltage at the peaks of the wave is 1.41 (aka, the square root of 2) times the RMS value. This is the voltage that the filter cap would get charged up to in an ideal rectifier. The diodes act like little one-way valves for electricity (in fact this is why us Brits call tubes "valves") so the filter cap ends up charged to the highest voltage that the AC ever reaches, simply because current can get into it through the diodes, but can't get back out again. (except of course by flowing through the amp circuit and operating it)

In real life, some of the voltage gets wasted in the rectifier tube, and in the impedance of the PT's windings, according to the rules of thumb that MWJB quotes. But the maker of the PT adds a few extra turns to roughly compensate the losses in the PT itself. And, the filter caps drain down between peaks as the amp uses current from them. All in all, it can be hard to predict exactly what B+ you'll end up with.

I never bother with tube rectifiers in my amp builds.

[Bassist001]

Wow thank you! Once again amazing info! Ya I was thinking that the 5v was a heater but just got confused when I saw the power going to the filter caps come off the heater ha ha I since I'm not an engineer I must ask more while I'm here if you don't mind but want effect does the size of the filter caps have? And same with the cathode caps? Thanks

[MWJB]

Bigger main (50-100uf) & screen supply (<40-50uf) filter caps will give you better dynamic response and the amp will hold together better when pushed. For an amp running much over 400vdc it is usual to wire the main filters as a totem pole (2 caps in series, each cap bypassed with a 220K 2W resistor), if the amp has a very high B+ (480+) then do the same at the screen supply. There is no advantage to using very high value filter caps in the preamp & PI nodes.

Larger cathode bypass caps promote more low end, watch for "woofiness".

[mooreamps]

You also need to consider heater current, 6L6 draw 0.9A each, EL34 draw 1.2A each,
As long as you are not building something too outlandish, somebody probably already makes your PT.

Forgive the intrusion. I thought the EL34 filiments would draw more along the lines of 1.5 amperes.

http://lib.store.yahoo.net/lib/thetubestore/JJ-E34L.pdf


-g

[MWJB]

I stand corrected, SED even spec theirs at 1.6A.