Amp project power supply help, Big stomp box...

[57champ]

So here is a link to my build a spitfire like amp project image. https://img.skitch.com/20120117-frtf7i9mtw21fukstny98afmgd.jpg.  I am taking a Hammond L100 amp, and converting it to a spitfire while keeping the spring reverb drive and recovery circuit intact. What I can't seem to figure out is how to rebuild the power supply to get the right B+ voltages as the spitfires are much higher. The original power supply is on the left bottom, I want to keep the same layout just modify the resistor values. If I knew the idle current that my amp will draw this would be easy, but all I have is the info on the old power supply schematic. Any help on how to find out what resistor values to use would be appreciated. Oh and If it is needed here is the L100 amp I'm coming from https://img.skitch.com/20120117-q6csk5g87k6uxajucek3f8bhq3.jpg.

[diagrammatiks]

I can't tell what's between the rectifier and the first node that's 340.

but it should be possible to get 350-360 out of that transformer with a tube rectifier.

you'll need to be a choke to put between the plate and screen nodes.

for the dropping resistors you can calculate it with ohms law.

each 12ax7 will draw appx 2.2ma of current.

[57champ]

Ok so heres what I came up with, using 1.2mA as the current for each triode in the 12ax7s, 11.5mA for each triode in the 12bh7, and then 92mA for each 6BQ5. (all looked up on duncan amps tube data sheet and using Ia from the application data)
I get: 2.4+1.2+2.4+23+184=212.8mA, Does this seem like a feasible current draw for this schematic?

[amptramp]

According to the RCA Tube Manual, the absolute max cathode current is 65 mA.  Are you sure your current is quoted for one tube or a push-pull pair?  The RCA Tube Manual shows a maximum plate current of 92 mA for two tubes and screen grid current of 22 mA peak, also for two tubes.  This is for operation in Class AB1 at 300 volts.  The 184 mA component should be 114 mA.

[diagrammatiks]

you'll be fine. you're not even adding that much stuff.

[57champ]

According to the RCA Tube Manual, the absolute max cathode current is 65 mA.  Are you sure your current is quoted for one tube or a push-pull pair?  The RCA Tube Manual shows a maximum plate current of 92 mA for two tubes and screen grid current of 22 mA peak, also for two tubes.  This is for operation in Class AB1 at 300 volts.  The 184 mA component should be 114 mA.

you know what your right the 92 mA was for a AB1 push pull pair so that makes my new current draw 212.8-92=120.8mA, seems kind of low for this amp.

[amptramp]

I get 143 mA.  2.4 + 1.2 + 2.4 + 23 + 92 (both 6BQ5 plates) + 22 (both 6BQ5 screens) = 143 mA.  This seems realistic if your plate voltage is 300V.

[57champ]

So I'm really wondering what the process of designing a tube amp power supply looks like, is it normally just find a ballpark value for the current draw and a starting voltage after rectification and then use ohms law to find the resistor values needed for the right B+ voltages. It seems like it should be more cut and dry. Well anyways given this is the right way I can't figure out how to get 352V after the 5U4. The circuit here https://skitch.com/70rhodes/g397x/l100-power-supply says the current for the original organ amp (which was also running a few more 12ax7s and 12au7s) was 60mA. So theoretically the 64ohm resistor right after the 5u4 should only drop .060*64=3.84V which puts me at 340+3.84=343.84V as my starting voltage, I need to start at or above 352V. The 5U4 drops 44v at 225mA could this be the reason I don't have a high enough voltage? I'm stumped, I just want to get the right voltages so that I don't have to attempt to re-bias the amp because the voltages are drastically different or have it decide to ruin a perfectly good pair of tubes when I turn it on.

[iccaros]

352 or 343, what is the differences? your within 10% and you will not hear it, its too close
the voltages given are what the designer viewed or calculated under perfect conditions, all based on what you think come from your wall. My wall puts out between 119 and 125 depending on time of day and load on the system. We do not have  a great grid out here. but that change in voltage will change what you read depending on time of day and where you plug into the grid.
What you what to concern yourself with, the ratio of change between stages, what is the percentage of voltage dropped.

if your concerned about the bias of the tubes, than 1) put in place a way to adjust that or 2) change the design to not pull more than 80% current, but using just the design and hoping to get perfect voltages is a fools errand. 
If you are using an old organ transformer, you have a good change of limiting the life of the tubes because the heater voltage will me high. Most were designed for 110 or 100 vac in the states and we now have 118 - 125. this may shove your heater voltage to over 7v which will reduce their life.

[57champ]

Thanks I'll put my time into getting the voltage drops right.

you'll need to be a choke to put between the plate and screen nodes.

Oh, and the original has no choke between the screens, I really don't think this is needed, It would just help reduce ripple; maybe regulate the power supply a bit better but since I don't have room on the original enclosure I don't think I am going to add a choke. Correct me if I'm way off base.

[PRR]

Your plan is essentialy the SAME as the Hammond, as far as power consumption. It worked for him, it will work for you.

340V is just fine. It is already at the high side of 6BQ5/EL84 ratings, 350V is, strictly-speaking, "excessive".

Yes, commercial and hot-rod _guitar_ amp designers often cheat-up the supply voltage past the manufacturer's ratings. The specs will give a good long life and low cost of use. Musicians don't play 24/7, don't pay a tech to replace tubes, do have other expenses much greater than tube costs, sometimes get paid... a couple more watts seems justified even against more frequent re-tube.

You do not have to re-bias, the proposed plan is self-bias. You can change B+ from 200V to 350V and the current will change in proportion, and stay in proportion to the load impedance; only the heat and maximum power output change.

200V will never kill a self-biased EL84. 400V won't kill it quick, and then from dissipation (red-plating) not over-voltage. 350V, 330V, 352V... these are No Difference in tube work.

[amptramp]

The 6BQ5 has a design centre rating of 300 volts so with self-bias of 11.8 volts, it can go to 311.8 volts on the plate, but tubes can and do survive overvoltage and most guitar amp designers put excessive voltage on the tubes with a reduction in life but this is usually not that objectionable.  However, with tubes becoming more scarce and more expensive, it may be preferable to reduce the voltage and extend the life.  You should also check the input voltage requirement.  The hikes in line voltage since the amp was designed will add stress.

[PRR]

> tubes becoming more scarce and more expensive

Adjusted for inflation, compared against hamburger or gasoline, tubes are cheaper than ever. The popular types are not scarce, and arguably ordering via iPhone is easier than driving to the TV repair shop was. I expect EL84 to be around a long time.

> design centre rating of 300 volts

Design MAX is 330V. "Center" is for when you won't check, such as making a million radios. "Max" is when you keep a sharp eye on things, such as DIYers who use a volt-meter. Allowing ~~5% for bias and losses gives 345V total B+.

In fact many modern EL84 are built to stand the higher voltages used in many beloved amplifiers. However self-biased EL84 amps with reasonable load impedance will over-heat or distort on voltages much higher than the conventional 300V supply.

[diagrammatiks]

Thanks I'll put my time into getting the voltage drops right.

you'll need to be a choke to put between the plate and screen nodes.

Oh, and the original has no choke between the screens, I really don't think this is needed, It would just help reduce ripple; maybe regulate the power supply a bit better but since I don't have room on the original enclosure I don't think I am going to add a choke. Correct me if I'm way off base.

the matchless has a choke. you definitely won't get the right screen voltages without one.