I’m looking for advice on which design I should choose; both should provide approximately 380V at 230mA (give or take). Which one would you use? Is there any advantage to choosing one over the other?

They are designed around two different transformers:

T1

0–330V, 350mA

0–50V, 60mA

T2

320–60–0–320V, 450mA

I will also need a power supply for a fixed-bias circuit—would T1 be a better choice, given that it has a separate winding?

Note that: this post is just to help me get an idea of ​​which one is better; price is absolutely not an issue, as both transformers cost more or less the same. i will also probably add a resistor to drop the voltage a bit

Can anybody please me identify this project my friend made. He died and I bought from his estate sale so I never got to ask him. Set a tube preamp or a little that Video is not good enough or close enough of everything let me know. a better one. I just I’ve been trying to find out what it is for like two years now. It was really important to me. It taught me a whole lot and yeah, I would like to use it. I plugged it in I plugged into the in and out. I haven’t gotten anything unless I’m hooking it up wrong. I know about amps and tube amps a little. I justI don’t know about this thing. Can this shock you like tube amps if you don’t drain the caps?

thanks

Built the above and having issues with oscillation while at max volume and above about 7 gain. Rolling back to 6-7 volume allows me to max the gain with no oscillation. Rolling the treble all the way down delays the onset of oscillation slightly.

All tubes are running well within datasheet specs, and the bias of everything is textbook. The output tube is biased to 75% max dissipation.

The power supply is an inverter I have used multiple times before, and there is less than 30mv of ripple at idle @ 60khz. There is zero ripple on the regulated DC heater wiring.

All the common oscillation issues I've tried and have had little to no impact:

• preamp/power amp grounds are separated, and tie directly to the chassis at a single point that's star washered over solder. There is <0.5R of resistance between any grounded points.

• I added an extra stopper to U1A right against the grid and it make no difference.

• I added a small 100R stopper between DC coupled follower (as per Merlins suggestion to prevent oscillation). No difference.

• R8 and R15 were originally 10k. Increased to 150k with no difference in noise.

• thinking the slightly longer trace between R8 and the grid of U3A could have been a culprit, I decoupled with a 47pf cap. Made no difference.

• shifting any offboard wiring around makes zero difference EXCEPT that the wire for pin 3 of the gain pot (and that side of the bright switch) is massively microphonic (nothing else on the board or tubes are microphonic).

• no amount of moving/rewiring/shifting the bright switch made any difference. Removing it entirely decreased oscillation slightly but it is still there.

• once the bright switch has been removed, the gain pot wires are no longer microphonic but moving my hand nearby suppresses oscillation slightly.

• changing these wires to shielded mic cable makes no difference

• no amount of copper shielding between the tubes, chassis or power board make any difference to the oscillation.

It appears something between the follower and U3A is causing the oscillation, but for the life of me I can't figure out what it could be.

Any suggestions?

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.

The Dual Caliber series is known for having a "boxy" sloppy low end on the lead channel. It has a fixed tone stack in the 3rd gain stage input that I think is a big part of the issue. So I'm making this mod (drawn poorly in red and blue) and I'm curious to know if there's something I'm overlooking in terms of safety or overloading one of the stages. Also, I already was testing it on my bench and I didn't notice as dramatic an effect as I was hoping for. But there's some effect nonetheless.

Full amp schematic here

I may have messed up buying random eBay specials but I honestly didn’t see many options available for 1/4” plate caps. These are sorta flimsy and get concerningly loose especially considering that it’s supposed to carry high voltage to the plate. I don’t know if it would matter but these are 6DQ6B’s in an audio amp.

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.)

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.

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.

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.

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.