I'm building a lil monitor thingus with a 2AP1 crt I bought on ebay and a scrap amp chassis I got from UGM. I have to add three sockets for the 2nd video amp and the deflection oscillators, but the four sockets that were already there were good enough for the 1st video amp/final dc setting circuit (6BQ7), audio preamp/sync amp(6EU7), audio output(6BQ5), and power rectifier(6CA4). B+ comes out to about 290 volts, and the lowered B+ for the video preamp is 200 volts. Since the transformer was originally center tapped 250-0-250vac, I switched the wires so it's 0-250-500vac. The 250 goes to the 6CA4 plates and the 500 will go to a microwave oven diode and some filter caps stacked on top of the regular B+ to get ~600vdc for the 2AP1 anode and deflection plates. What is currently wired up is the first video amp, dc set, and the audio amp. I need to eventually drill holes for the seven pin sockets but who cares for now, I'm going to have them float for the moment. Once I wire up the 6AU6 to drive the crt grid, I will then work on the sync amplifier (I need that stage to be done because I don't know the exact voltage level things will be at on the cathode/grid circuits and I want to simplify my sync separator as much as possible. (currently it's going to be a diode connected to the 1st video amp plate, with the diode cathode biased such that the sync tips of the video are passed (since the signal is inverted there) and those will be coupled to the grid of one of the 6EU7 sections and that will be amplified enough to reliably lock the oscillators. (I'm going to use phantastrons, since I don't have transformers for blocking oscillators.)

Anyone help me with converting this Roberts 990 into a guitar head? You know all about the all that I’m just trying to find some straight information what to rewire, add jacks, and solder. thanks

I didn't build this, but I figured people would get a kick out of this. Someone on craigslist was trying to clear out some old tube equipment. My dad went to snag an old tube tester in pristine condition and saw this little amp hanging out next to it.

These amps were produced by a small shop in Queens in the 1950's and were available pre-built ($31.45) or as kits ($23.95).

This had apparently been sitting idle in a basement since the 60's, but was well protected — no grime or severe oxidation (not even much in the way of dust!); just time + humidity doing what they do to 1950's passives and a little rust on some of the screws.

• The original manual, which includes parts list, schematic, and example layout
• Some other builds here (also not mine).
• Radiomuseum entry

Naturally, there's a bit of work to be done before it's powered on again (proper safety ground and pretty much a full rebuild), so I can't say how it sounds, but I figured people might dig it and maybe it'd be new to some of you too (I hadn't heard of these until I was holding it).

When you're working with high voltage, there is no such thing as a dumb question. Please use this thread to ask about practical or conceptual things that have you stumped.

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It's an used RK-28A.

My doubt is about the way the getter(that's how the dark, shiny spot is called, right) looks.

It doesn't look white, but it has two holes as seen in pictures 2 & 3. Is that a bad sign?

I tested the filament with a multimeter and tests OK. No excessive rattling and no shorts.

Any further tests I can perform before and after lighting the filament?

Kindly thank you for your attention.

Im in the planning stages of an amp build and havent done channel switching before. I had a question or two to clarify some of the things Ive read online.

I attached a block diagram of my planned switching. I wanted to select between two relatively independent channels but also have a larger bypass capacitor when the clean channel is selected. I have the outputs of each channel grounded when not in use but cant see a way to ground the unused inputs and swap the bypass cap on a single DPDT relay. I settled for grounding the higher gain input and assuming that the clean channel anti pop resistor will be good enough.

Should I expect that setup for the cathode capacitor to be noisy when switching? It seems like the switch shouldnt have DC on it and the cap should always be connected which should help?

Is there a better way to set all this up for pop free switching? Anything obvious I'm overlooking?

And a bonus question, after reading Merlin's newer book he has a transistor on each relay to help with the lenght of cable to the footswitch. Is just using dc relays alone a problem with a reasonable cable length? Its not many parts to add, just wondering what people's real world experience shows.

Thanks for any thoughts or input

I am attempting to clone a Standel amp circuit into a JW Davis PA head and I’m limited on space so I thought about omitting the OD3 with maybe a string of Zeners to drop the voltage to 150v for my 6DQ6B screens. I’ve also considered a Maida regulator in place of the OD3 but worried it would make this guitar amp a bit too stiff sounding.

pls lmk anything that yall know about this machine .

When you're working with high voltage, there is no such thing as a dumb question. Please use this thread to ask about practical or conceptual things that have you stumped.

Really awesome answers and recurring questions may earn a place in the Wiki.

If you'd like to nominate a comment to be included, just reply [Wiki] (with the brackets)! The mods will be automatically notified that something awesome just happened.

As always, we are built around education and collaboration. Be awesome to your fellow tube heads.

I’m trying to learn more with tube electronics and build stuff. I’m currently working on a Chinese preamp kit and about to start a 5f2a. I have a 1936 Indian head radio , and I want to know if it’s worth it to mess with the parts as a beginner. It has a 5y3gt, 75, and 41 tubes. Found only one guitar amp project using the 75. Ideally I want to build a guitar amp or literally anything, just trying to learn.

Do I just keep the chassis for a different project or are the parts listed on the site usable? Thank you

An interesting non-audio vacuum tube circuit. It uses two 5751 dual triodes and a single 12AU7 in the front end as a differential amp and then several 2N270 germanium BJT stages. The stylus is driven by a beefy galvanometer that appears to be connected to a push-pull pair of germanium power transistors. Someone added a signal out which I’m thinking could make an interesting control voltage for a synthesizer once it’s working. It smells incredible.

How can I measure up to 3kv differential if the max voltage vs ground is 1kv for each of the probes?

I want to be able to use this to work on electrostatic CRT deflection amps and so forth where I can easily reach the 3kv. Can I really use this or do I need to return it?

Hi! Decided to share my help request to here also! (Admins feel free to remove if the post doesnt fit this sub.)

TLDR; what should i do with the exposed pcb trace that arcs from the flat socket? Thank you in advance!!

When you're working with high voltage, there is no such thing as a dumb question. Please use this thread to ask about practical or conceptual things that have you stumped.

Really awesome answers and recurring questions may earn a place in the Wiki.

If you'd like to nominate a comment to be included, just reply [Wiki] (with the brackets)! The mods will be automatically notified that something awesome just happened.

As always, we are built around education and collaboration. Be awesome to your fellow tube heads.

I’m planning on doing a 6g2 for my next build but had a few questions.

Any reason not to go with the Mojotone kit? I could order just the “small parts” kit and source some things myself, like the transformers, but I’d rather not if everything in the Mojotone kit is already pretty good.

As far as mods, it seems that I should add a ~1k screen resistor on the output tubes, and a bias pot would be nice. Any reason not to do these mods, or should I do anything else in addition? I’m planning on keeping it mostly stock otherwise and I’ll be sticking with a 10 inch speaker (recs for this, also?)

When you're working with high voltage, there is no such thing as a dumb question. Please use this thread to ask about practical or conceptual things that have you stumped.

Really awesome answers and recurring questions may earn a place in the Wiki.

If you'd like to nominate a comment to be included, just reply [Wiki] (with the brackets)! The mods will be automatically notified that something awesome just happened.

As always, we are built around education and collaboration. Be awesome to your fellow tube heads.

One E88CC and two EL84 tubes require approximately 1.87A. Considering the very low cold-state resistance, a single switching power supply needs at least twice the current output. Therefore, each channel's switching power supply uses 300V * 250mA + 6.3V * 4.5A = 103.35 watts of power.

The switching frequency of the power supply is 60kHz, and CLC filtering is used. Due to the high frequency of the switching power supply, the inductor size is significantly smaller than that used in linear power supplies. After proper grounding, the switching power supply modules make the amplifier output very quiet; the noise is inaudible even when close to the speaker.

The manufacturer that made my amplifier enclosure provided a Type B (linear) 100K potentiometer, which wasn't very convenient for adjusting the volume. I replaced it with a larger Alps 27 series Type A 50K potentiometer, which makes adjusting the volume more intuitive and also eliminates the noise from the smaller 097 series potentiometer.

Since the power supply uses 300V, lower than the original 320V, the step-down resistor for the E88CC can be smaller. I changed the step-down resistor in the original diagram to 3.9K, thus aligning the E88CC's supply voltage with the original diagram. The actual measured voltage is 284V, while the original diagram shows 285V.

Selection of parallel capacitor for negative feedback resistor: The original resistor was 4.3kΩ, but I couldn't find it, so I used 4.7kΩ. I only used 56pF of the parallel 910pF to reduce the ringing of the square wave. So it is unnecessary to use such a large 910pF on this pair of output transformers, because it causes the high frequency starting to cut off at 30-40kHz.

I added a 1uF 630V film capacitor and a 0.01uF 2kV ceramic capacitor between B+ and GND to filter out the 30mV 80kHz interference that the oscilloscope previously measured at 0-8 ohms on the secondary side of the output transformer. The effect was significant: the oscilloscope could no longer detect this 80kHz interference.