Building a high power audio amp from RF power tubes - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: July 24, 2022, 01:32:39 pm
content: tubes like the GU-33 are quite cheap unlike for example 810's... I feel pretty certain that it is possible to make quite a descent audio amp from one of them, especially the GU-33 who'ts parameters are quite good for audio.but I have no experience with power RF stuff... maybe I need to add some extra anti-oscillation components?here's a rough design I made:[ Attachment Invalid Or Does Not Exist ]I know I know, the power supply is an abomination. and sorry about the messy connection dots, easyEDA isn't the best circuit designer.but what about the amp part of it? I chose the cathode and screen resistors just randomly, since I barely found any detailed specs on the tube, just a plain grid line chart curve image.and if my calculations are correct... it would deliver around 350 watts in class A, biased at around 150mA per tube. and 1.3kW for class AB2? but that would need to have G1 go up to 20volts, but at what voltage will the grid exceed the maximum rated "2 watt dissipation"?oh and I just realized that I can't put the output tube filaments in series like that because one of the pins is hard wired to the cathode pin.

author: TimFox
author_rank: Super Contributor
timestamp: July 24, 2022, 02:08:42 pm
content: Do you have a source for the required output transformers?

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 24, 2022, 02:35:06 pm
content: Hi ELS122,First of all - do you have the fitting sockets?What output-transformer will you use?350W Class-A = 1.4kW Heat! Is electricity still that cheap for you?You connected the first GU33-pair Grid1 to 50V. Strange driver, why not an interstage-transfo?Datasheet:http://frank.yueksel.org/sheets/018/g/GU33A.pdfandhttp://frank.yueksel.org/sheets/018/g/GU33B.pdfMaybe consider an amp with GM70 (plenty of designs around):http://frank.yueksel.org/sheets/084/g/GM70.pdfor cyrilic:http://frank.yueksel.org/sheets/018/g/GM70.pdfAnd the sockets can be DIYd easy.Good luck - stay safe!

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 24, 2022, 03:12:41 pm
content: Holy Tinnitus Batman!That is well detailed work.I suggest to check the following 700 series Hammond HT transformers, which have a sub-range with 220 V 50/60 Hz primaries.You would need (3)https://www.hammfg.com/electronics/transformers/classic/700The restored amp here has pair of 7027A by JJ of Slovakia.530 V, Quiescent current is 55 mA/side.Tha amp gives 72 Watt output, but the standing power is 155 Watt incl fil +IqThe original 530 V rail was 2 by 40 uF in series, which I replaced by 2 higher uF ( can't find value ) in series, with 1M Ohm balance .The class AB should have rejection of ripple, and I would question whether you would need any filtering on the ouput tube HT.Note your voltage mutiplier will draw surge current on power up probably will trip a residential breaker.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 24, 2022, 05:14:46 pm
content: Quote from: ELS122 on July 24, 2022, 03:33:35 pmBut you show 3 phase uncontrolled rectifiers.The DC without filtering should have only about 4.5% ripple, so the average DC will only increase by about 2.25% with a capacitor filter. The trough of ripple is about 2.25% below the average. These are not like single phase rectifiers which have trough dropping to 0 Volt.Believe me, I am old enough to have done some 3 phase bridges supplying 50 kW triodes. They had no filters except RF bypass.Read about it and you can model a section of your multiplier.Add: read about the ripple and see waveform here: ( for example)https://www.sciencedirect.com/topics/engineering/three-phase-bridge-rectifierAdd2 : Further, my ref book says about push pull:"..push pull stage is unaffected by spurious voltages.. in supply circuit"" push pull places constant load on HT supply..as far as fundamental components of anode current are concerned.Those points , in combination with the "pentode-ness" of your tubes makes me think that DC filters may not be needed.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 24, 2022, 06:22:50 pm
content: I have not seen your rectifier circuit before, but it appears to use + & - of the 3 phases, so I conclude 6 pulse rectifier type.There is a standard 3 pulse rectifier but fell out of use because the ripple is higher, and line harmonics are increased, also core flux in transformer.Some of the rectifiers I did had 12 pulses and 24 pulses using phase displaced transformer windings.That is done not for ripple reasons but because higher pulse number > more cancellation of the lower harmonics in the incoming supply.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 24, 2022, 06:56:11 pm
content: I don't recognise the term "MOT"As others on this eevforum have mentioned, I really don't recommend proceeding until an isolation transformer is included.Even though the speakers are isolated, your controls and inputs are on circuits not galvanically isolated from incoming supply.Returning the HT DC via earth-ground is not allowed in most countries - an isolated bridge solves that.Also using all those electrolytics unfused on the HT multipliers may be a problem.You might find a small 3 phase isolation transformer new or used or use 3 of 1: 1 etc.You might find some lower impedance tubes down around 600 or 800 Volt HT that allow use of transformers like Hammond.And as TimFox mentioned it all depends on the possibility of getting an output transformer of correct ratio.Actually that is the starting point unless one is custom made. The tube selection might have to be based on existing transformers.

author: gbaddeley
author_rank: Regular Contributor
timestamp: July 25, 2022, 12:36:20 pm
content: MOT = Microwave Oven Transformer. These are not designed for continuous use, and get very warm, even without load.Start with smaller tubes eg. KT88 to get some experience. Then you will realise how expensive it’s going to get to scale up, particularly with output and power transformers. Look at other builders using 813, 833A.

author: CaptDon
author_rank: Super Contributor
timestamp: July 25, 2022, 01:00:11 pm
content: That style of grid structure doesn't like any grid current therefore the very low grid dissipation rating. You would be far better off building with some of the 'zero-bias' power amplifier tubes like 3-500Z or 811A. All they need is a correct plate voltage and impedance. With push-pull you can find sensible output transformers. This style of tube amplifier with 300 to 800 watts of RMS (with no headroom) was popular in commercial applications for laboratory amplifiers for plasma studies and so forth. They also saw use as A.F. modulators in A.M. broadcast transmitters. I don't see any practical application in audio for GU-33B type tubes. If you want to go that route there are hundreds of new and used 4X150A tubes available cheaply and the sockets can still be bought new or used, with or without the screen bypass built in. And a 4CX250 or 4CX350 upgrade are plug and play with a bias adjustment. So are the 8920 (or was it 8930) and the AX393 versions.

author: eugene
author_rank: Frequent Contributor
timestamp: July 25, 2022, 04:05:10 pm
content: It sounds like you do have some experience designing and building tube audio amps. So I won't try to steer you away from the GU-33. Besides the fact that it doesn't have a pretty glow to look at, it should be a fun tube to experiment with. It should bias up at a convenient negative grid voltage.I have no experience with transmitting tubes that were NOT in glass bottles. Not being able to see the plates might have the disadvantage that you won't know where to bias them. The older tubes all need to be run with the plate glowing cherry red. That's how everybody always biases them (assuming they do it right.) That's true whether used in a radio transmitter or an audio amp. Adjust the grid voltage until the plate is the right color. The graphite anode does the gettering when hot. It could be these tubes are designed to be run differently, I don't know. It's a 150W tube, meaning the anode is designed to dissipate 150W, so you could monitor anode voltage and current to achieve something approaching 150W dissipation.The way that *I* would want to do it, experimenting with an unknown tube like this, is to have adjustable anode, screen, and grid supplies. Being able to adjust the anodes, screens and grids with the twist of a knob is part of the experimental method of building an amp that sounds good. It is about sounding good, and experimenting, right? They way *I* would accomplish that is to go out to the shed and find three suitable variacs and three suitable power transformers, then build three separate power supplies.Are used 3 phase variacs common in Latvia? I can only dream about 3 phase power supplies in the US. 3 phase power exists for industrial uses, but it very rare in homes. The 3 phase variable transformers that I see are just 3 single phase units stacked with a common knob on top.In a project like this, the power supplies (including the filament supply) are often the hardest part. Once you have those, mounting 4 tubes to a piece of wood and wiring it to an output transformer is comparatively easy. Likewise the input and driver stages. Building everything as independent parts ends up being tremendously useful when it comes to experimenting. You might decide you don't like the emitter follower driver and want to try an interstage transformer. Or whatever. Maybe a different input tube. That might happen early, or it might happen after you've already been enjoying music through the thing for 6 months.You should count on trying different values of grid and screen resistors. Nobody here or anywhere can reliably predict if 47 ohms is the right amount. My experience is that lower grid resistance usually sounds better. But how small is too small?It is likely that the right output transformer would be difficult to find as an existing part. But, I can think of at least three places in the US that I could call to have one custom wound for a reasonable price. Not sure what exists in your part of the planet, but it might not be difficult to have one wound or even difficult to pay for it. Of course you would need to have specs. Nobody can create something custom without some specs to work with.

author: gbaddeley
author_rank: Regular Contributor
timestamp: July 26, 2022, 10:25:48 am
content: It’s hard to add avoid grid current when driving RF power tubes to decent audio output powers. How are you going to do that? Or is distortion not really an issue?

author: coromonadalix
author_rank: Super Contributor
timestamp: July 26, 2022, 11:05:05 am
content: check carver silver 7 and silver 9 amplifiers w tubessilver 7 schematics are on google .... the transformer was made with flattened / squared wires in it. to cut losses ...https://elektrotanya.com/carver_silver-7t_9t.pdf/download.html#dl

author: coromonadalix
author_rank: Super Contributor
timestamp: July 26, 2022, 03:33:07 pm
content: no problemosome schematics 200 watts and one 300 watts ??I don't think you realize where your going ... but good luckI had one friend who could design what you need in one day, he built 2x mono block amp of 500 watts each, I've seen them, BUT boy they heated a lot, he had to redo the house main electrical box. loll

author: CaptDon
author_rank: Super Contributor
timestamp: July 26, 2022, 06:28:29 pm
content: I think I have seen the book from Eimac (Foremost manufacturer of power grid tubes back in the day) called "The care and feeding of power grid tubes" as downloadable text on the internet. Their original designs were power tubes used in pulsed radar where the grids were pushed into the amps of positive grid current. After WWII they began to focus more on high gain high power broadcast tubes. High gain and high frequencies were obtained with very fragile grid structures. I forget which tube it was 4CX1000 or maybe 4CX1500 was a huge tube with over 1000 watts of output available and a G1 grid NEVER to exceed 1 watt of dissipation. Eimac who was a pioneer in 'external anode' tubes such as the 4X150A and its bigger brothers notes that most of the tubes of that physical make up are not suited for grounded grid use and not suited for positive G1 grid voltages and current. On the other hand, their spinoff of original 'radar' tubes starting with the small 4-65A and going up to things like the 3-1000 triode or 4-1000 tetrode are designed for G1 grid current and being pushed hard in class C R.F. amplifiers where grid current is the norm.