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So I'm guessing that this is changing how the tube is conducting when you ground it out. Still quite messy but i definitely feel it has more potential to grow Attached Files. I have attached the Vox AC15 schematic I am following.
Here's a couple of pictures of the amp as it is right now. The original circuit didn't have a filter choke so I have added a nice vintage partridge uk made After testing the psu section for correct operation I moved on to the preamp stage, starting with the ef86 channel of the vox ac15, the so called normal channel. Any common cathode gain stage needs the grid connected to ground by some resistance.
Posts Latest Activity Photos. If you need more distortion, you could lower the plate resistor value. I found out i was missing the 1m grid leak resistor, hence the different readings with input shorted to ground. I am studying more circuits, mainly early mesa boogie stuff, but i don't really know if these preamp stages could match well this vox-like circuit.
Thanks for your reply I believe the AO is different in the ef86 stageas the resistor values are different from what i had in my amplifier, which basically had the same resistor and capacitor values as the vox ac15, minus the cathode bypass capacitor, uF. Any suggestion would be highly appreciated. The original circuit used pretty much the same capacitors and resistors values so i didn't need to change much here. I suspect as soon as the power stage is working the voltages on the ef86 will change again I think that I will go ahead and wire up the power stage.
But with the correct input configuration i still have the screen grid at v and the plate at v I have done some experiments with different plate and screen grid resistors With k instead of k plate resistor the voltage on the plate goes up to v with the screen grid still at v Went back to the stock k and changed the 1m screen grid resistor to 2m. Without the tubes in circuit I have measured around V on the preamp tubes anodes With the ef86 in circuit i get the following readings plate pin6 50V cathodes pins 3 and 8 3. Highly recommended. I'm saving this one for later.
The screen grid is about 20V higher than the plateand you totally make a point stating that should be the other everett escort ao around. Log in. So my thinking is this is only at DC idle condition that the screen is higher than the plate and then under load, the screen will drop to below the plate as the data sheet and Valve Wizard site shows. It sounds like you've gotten a good start but maybe this will help some more.
I had been really missing a vox sounding amp in my setup so I decided this amp would have been perfect as a conversion experiment. Some advice would be helpful in order to get to the final result without damaging components, especially the transformers. Forgot password or user name? You could then mix the channels with kk resistors like the attached Fender schematic with them highlighted in yellow. The original schematic you posted has this configuration for the second channel.
I've also noticed another thing, if i ground the input the voltages go up to V for the screen grid and V for the plate. Too many other projects right now to finish.
I would think your Mesa Boogie preamp would work fine if it has two 12AX7 gain stages that are cascaded. Notice in the Vox schematic how the input grid is connected to ground via the 68 grid stoppers and that 1 Meg grid leak resistor.
When cranked the distortion is very smooth and thick, definitely what you would expect from a vox.
I started by carefully studying the vox schematic and comparing to the circuit, taking out some components that weren't needed and then wiring up the power supply circuit with new filter capacitors. I finally got some time to work on this project. It goes on to say that the 1 Meg resistor provides the necessary voltage drop for a suitable working voltage since DC screen current flows in this resistor. On the old 18 watt forum a lot of people were adding EF86 preamp tubes at one time.
See one. Tags: None. Pretty sure you want the PDF file as there are several to chose from.
I found an interesting item in one of my books when doing more research on the EF It had the exact input of the Vox AC 15 as the example and it showed a supply of volts, plate resistor of k, a screen grid dropping resistor of 1 Meg. It showed the plate at 81 volts and screen at 99 volts. Comment Post Cancel.
As you confirmed I have more or less the right supply voltages so I hope I won't be burning anything. of 1. I couldn't find any schematic for the original circuit, but I could see many similarities with the Vox ac15 circuit. Filtered by:.
Also you could lower the cathode resistor to bias it hotter and possibly add a bypass cap to the cathode resistor if there are any that are unbypassed. Originally posted by beatnik View Post. I found out to be quite different from the vox schematic i am referencing to.
Hello everybody, I got this tube amp chassis unit, model AOC. I found information that it was an Hammond produced amplifier mounted in an Everett organ. Today have been tidying up the wiring and doing some more checking, then fired up the amp Passed the smoke test and the voltages around the tubes looked fine.
One other thing, I haven't done anything to my AO yet, so I can't speak to experience with it. Do you have the grid referenced to ground right now like the schematic shows? The EF86 is quite microphonic but i have a handful of these and will try them out to find a good sounding one without too much noises I am now thinking about the next step.
Is this normal at all? Plate load and screen grid resistors were definitely k and 1m I double checked the wiring and measured voltages again. I see that the AO has a volt first node in the power supply and the Vox isso that should be close enough.
All rights reserved. When the amp is at idle, it's called DC idle static condition since there is no al applied.
This also had basically the same resistors values as the vox, but wired somehow differently so had to do more work Before working on the power stage I wanted to do some voltages tests with and without the preamp tubes. Then when al is applied, then it's considered dynamic AC condition which is different than idle.
Yes No. OK Cancel. I have four more noval sockets available on the chassis and I would really like to expand the amp adding some more flexibility Right now I am thinking about two things : an fx loop i use a lot of effects and a second channel, maybe with 12ax7 instead of ef86, and i would like this channel to be even more gainy and with more preamp distortion I would really appreciate if someone pops in and contributes some ideas. What could i try to get the right voltages on the ef86?
Done some guitar playing and I am very happy with the sound, really much better than expected! Then moved on the phase inverter stage.
If you look at this data sheet, it gives a typical condition as volts plate and v screen. I am wondering if this would be fine for the ac15 circuit or I should do some changes on the plate resistors? If anyone has more insight to add, it would be appreciated. template Next. So don't worry about the screen being higher than the plate.
Music Electronics Forum. Also, I think you want the screen grid lower than the plate on the EF The organ schematic shows 80 volts on the plate with a k load resistor everett escort ao a k screen grid resistor with a volt supply but doesn't give the screen voltage. I am not really a tube expert, I have some building skills but my theory and understanding of circuit operation is quite limited. I would keep the resistances on the EF86 as they are if you want a Vox.
Also, concerning your question about volts on the preamp tube plate pins without tubes installed; your voltages will come down as you load the amp with tubes, especially the power tubes. Very easy to understand with minimal math. I can't say for sure why the screen grid is higher than the plate. I must admit I don't have much experience with small al pentodes, just power pentodes where the screen is just a few volts lower than the plate even at idle.
Logging in Remember me. As an added item, that book about preamps on the Valve Wizard site is a very good book in plain, easy to understand English. It depends on what you want to accomplish. Very good bass response as well.
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