Messing around with tubes.( vacuum tubes ) - Page 1
Forum section: electronics / projects

author: Refrigerator
author_rank: Super Contributor
timestamp: August 21, 2016, 01:19:09 pm
content: I found myself a 6i1P/ECH81/6AJ8 tube and i wanna mess around with it.It's one of those niche radio transmitter tubes with a triode and a pentode(?) in a single envelope, so you know what, let's make a guitar amp out of it.It's a small tube so i don't expect to get much power out of it, that's if i get anything at all.I also found myself an audio transformer so that's nice.^^^^^^ not sure about pin 1 though, i think i'll leave it floating but i've heard floating pins on tubes can do bad thingsso i'd appreciate some input on that.Thanks.Ps: yes high voltage, i've been "tingled" enough times to know not to touch it.

author: Ian.M
author_rank: Super Contributor
timestamp: August 21, 2016, 01:56:44 pm
content: Wow its got a heptode in there. Seehttps://en.wikipedia.org/wiki/Pentagrid_converterYou've got two control grids, G1 on pin 2 & G3 on pin 7.Pin 1 is their screen grids, G2, G4 so should be pulled up to +B via a resistor.If its being run as an IF amplifier the Philips ECH81 datasheet says G1 (pin 2) is the input and G3 (pin 7) should be grounded, but it might be interesting to try it in space-charge mode with G1 as the accelerator and G3 as the control grid.However for your application, the Heptode section's got the higher plate dissipation rating (only 1.7W), so you'd want to use that for the power stage. Triode as preamp, couple the signal to G1, and maybe try negative feedback from a pot across the output transformer secondary on G2 to alter the linearity and gain.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 21, 2016, 02:37:33 pm
content: I did a little doodle of how i wanted to connect mine, which i based around the pentode guitar amp schematics on the net.However in my doodle pin 7 is pulled up through a resistor.I'll upload a pic so you can see what i mean.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2016, 02:55:12 pm
content: You can't get any more than headphones power out -- heptodes are notoriously inefficient. Only a fraction of cathode current reaches the plate. They're intended for radio mixer service, where the signal efficiency (mixer transconductance) is better than other types.Is this really the only one you have handy?Tim

author: Pjotr
author_rank: Frequent Contributor
timestamp: August 21, 2016, 03:02:35 pm
content: Wow, you want to hook up a loudspeaker to it?That ECH81 is a common receiver mixing radio tube containing of small signal triode intended as an oscillator and a heptode for use as a mixer.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 21, 2016, 04:24:51 pm
content: To me the tube is quite useless so i don't care if it goes bang.And if it goes bang then i can say that " I popped it ! ".I did get myself a variac so i can bring up the voltage, just so i don't pop the tube instantly.I don't think anyone has ever tried doing something like this so it will be lots of fun.And i will use a proper high fidelity speaker to test it, maybe whip out a few alnico ones if it ends up working.

author: Pjotr
author_rank: Frequent Contributor
timestamp: August 21, 2016, 04:51:53 pm
content: QuoteAh thus a burning/exploding guitar amp. With the cat as audienceOr heat is up to 600 degC and watch when it implodes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2016, 04:58:36 pm
content: That's not really one you can "bang" with... you'll maybe get a little spark inside it and that's it, failed shorted.The big glass ones (sweeps like EL519) can dissipate enough power to make the plate glow yellow hot, and heat the glass to the softening point.They aren't made with magic smoke, so there's still very little to release. Rugged beasts!Tim

author: Audioguru
author_rank: Super Contributor
timestamp: August 21, 2016, 05:28:02 pm
content: Your sketch shows a "negative feedback" capacitor from the speaker to the bypassed cathode that will not work since the cathode has a bypass capacitor to ground.Negative feedback reduces gain and reduces distortion. For a geetar amp you probably want LOTS of distortion and enough extra gain to make the tube clip like crazy.

author: oldway
author_rank: Super Contributor
timestamp: August 21, 2016, 06:18:01 pm
content: Look for ECL or PCL tubes, they are intended to do what you want.ECL86 is the best one (audio tube) but you could also use ECL81, ECL82, ECL85, ECL805.....

author: Refrigerator
author_rank: Super Contributor
timestamp: August 21, 2016, 06:29:09 pm
content: Just came back from the bench and i have good news!It didn't blow up yet, granted, i only did a simple power on, no signal in yet.And it's a mess i tell ya, quite the rat's nest in there.So i brought it up with the variac untill it stabilized and checked a few points.There are slight variations between my doodle and the prototype but i couldn't be bothered to find the exact value components.Measured about 1.6mA cathode current at 230VDC supply voltage, seems a little lowbut let's not go there yet.Now would be a good time to give it some input and see if there's amplification.Place your bets!

author: Refrigerator
author_rank: Super Contributor
timestamp: August 21, 2016, 07:19:39 pm
content: We have sound ! But, not much of it.Seems like there's no amplification going on rather the signal is almost the same but with less low end, sounds like my tube amp instead of making the sound warmer makes it cold.I can hear some clipping so i think the tube could use some more bias.There's also some hum, it's not terrible but it's there.I still have the unused triode that i'm going to take advantage of.Edit: oh yeah and i didn't connect the negative feedback from the audio transformer secondary, gain is low enough as is.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 21, 2016, 08:04:46 pm
content: So what did you end up doing with pin 1? It should be connected to the +DC via 20-50K.

author: Cyberdragon
author_rank: Super Contributor
timestamp: August 22, 2016, 04:56:58 am
content: That tube is totally not meant for audio service. If you get any decent power out of it it'll be very distorted (in a bad way I.E. heavy clipping). Maybe see if Dave can ship you those tubes he got in that mailbag, there was a 6AQ5 in there which is a speaker driver.

author: tautech
author_rank: Super Contributor
timestamp: August 22, 2016, 05:21:54 am
content: Ha, that's a modern tube.My very OLD RCA book doesn't list it but my old Mullard one does.6.3V heater @ 300mATypical use.Triode as oscillator 800 mW maxHeptode as mixer Na2W maxNg2+g4800 mW maxPlate voltages typically 250VNot fit for audio use.

author: Rbastler
author_rank: Frequent Contributor
timestamp: August 22, 2016, 05:30:10 am
content: You could use the ECH81 as signal generator in a Miller Transistron for example. A application for that would be that power supply tester:http://www.jogis-roehrenbude.de/Leserbriefe/H.M.Sauer/Netzteil-Tester/Netzteil-Tester.htmOr you build a Superhet radio with itAs a mixer tube, it's no use to pretty much anything else.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 22, 2016, 06:43:22 am
content: So i connected the triode as a preamp and it started oscillating, predictable, the pitch of the oscillation can be changed by rotating the tube, so that's fun.Good thing is that when you give it audio the oscillation stops and it starts working.And there's amplification, despite the dirtortion, which is not too bad.Although at one poind it did make clicky discharge noises, i replaced the poly caps with some HV ones and it's all hunky dory.Now i want to do something about that oscillation, maybe do something interesting with the way my audio transformer is connected, which should help out with the low plate current.I'm also not sure about that triode, i'll try a transistor preamp just to make sure the triode isn't ruining my precious sound quality.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 22, 2016, 06:44:25 am
content: Quote from: PA0PBZ on August 21, 2016, 08:04:46 pmI left it floating.

author: calexanian
author_rank: Super Contributor
timestamp: August 23, 2016, 02:45:17 am
content: You would have much more useable results with say a single 6v6 or el84 and a 12ax7, or even just a single 6BM8. I have even built a simple sub watt amp with a 6SL7 that worked pretty well.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 23, 2016, 03:16:50 pm
content: Quote from: calexanian on August 23, 2016, 02:45:17 amI had a bag full of those somewhere.

author: Vucinic
author_rank: Newbie
timestamp: September 07, 2017, 09:39:53 am
content: http://meatexz.com/engel-sound/ech83/ech83_preamp_sketch_engel_march2007.jpgYoucan use for a preamp

author: Molenaar
author_rank: Regular Contributor
timestamp: September 07, 2017, 11:17:02 am
content: Quote from: Refrigerator on August 22, 2016, 06:44:25 amYour gain should increase if you do connect it.

author: drussell
author_rank: Super Contributor
timestamp: September 07, 2017, 04:23:20 pm
content: If you want an audio power amplifier kind of range (like at least half a watt) from a pre-amp tube, you really need something like a 12AU7 or a 6CG7 or something that can actually do a little power. I really needs to be one of the low-mu triodes for it to be able to do any current when it is intended for low levels.Even better would be something like a 6/12BQ6 which is actually INTENDED to be a cute li'l power amplifier.

author: george.b
author_rank: Frequent Contributor
timestamp: September 07, 2017, 09:40:37 pm
content: Can't you find old soviet tubes there in Lithuania? The 6N2P is more or less equivalent to the popular 12AX7 (except for the filament voltage), but cheaper.

author: Rbastler
author_rank: Frequent Contributor
timestamp: September 08, 2017, 07:24:15 am
content: You could build something with a 1SH14B and a 1P24B, or DF91/1T4 and DL93/3A4. The are all small tube, with the 1SH24B and 1P24B beeing "pencil tubes". So no socket. The other two have a pico-7 socket.You may also came across the 6N8S. A small amp might be possible as well with such a tube. I only used it as a preamp tube in my EL34 PP amp tough.

author: Gyro
author_rank: Super Contributor
timestamp: September 08, 2017, 08:21:03 am
content: Quote from: george.b on September 07, 2017, 09:40:37 pmAgreed, there is a treasure trove of former soviet tubes avalable cheaply. The 6n1p is a good workhorse with reasonable anode dissipation (for a signal triode) it makes a good general gain stage and driver. Go for -EV versions if possible - the tougher, longer life mil spec versions.This is the go-to site for tube types and characteristics....http://klausmobile.narod.ru/testerfiles/