Broken Input Selector, Way to bypass?

[changedsoul]

Hi, Ive had this tube amp for a few years now and its got a broken input selector switch. I tried to just remove the thing and wire the amp up directly to one of the inputs since I dont have any records or old tape heads. Anyways, me being ignorant with this stuff with the added fact that I had no schematic at the time, I made a mess and the amp doesnt work at all now . So it sat for years untill I came across a schematic and noticed that what I thought I was trying to remove seems to have been needed in some way, but I dont know enough.

Anyways, Can someone give me some info on how I can bypass this selector switch? I'll just be using the aux input anyways and the switch is broke, so I am hoping I can just reattach the right wires where they need to go, but alas, I am clueless. I just know enough to solder and not kill myself while tampering. Not enough to follow the schematic in a way I would be able to figure this out.

Thanks, I would love to hear this thing operate again.

[loudthud]

Hard to know in advance how well this mod will work but it's worth a shot. The Stereo/Mono Function switch should allow you to route the signal to both channels. I bypassed the tape head preamp because I don't think you need that much gain. Let us know how it works.

[changedsoul]

Well In my ignorance back when I was tampering without the schematic, I snipped the switch out, along with the wires going to it indicated by my Blue arrows. The mod you suggested is what I actually tried at first and it failed. But after seeing the schematic, I see I made a mistake elsewhere hooking things up. My concern now is before I fix it according to the cartoon, where do these points go the blue arrows are pointing to.
Looking at the part of the switch that remains but not in circuit, I can map the side with the broken ring ( left part), but the one with the solid ring ( right part) seems like as you rotate the switch its grounding all inputs except the one selected? But i am un sure because it seems like there was more connections actually on the switch than what is being showed in the schematic.

So I guess my question is, the points shown by the blue arrows in the file I uploaded in this reply, from a amp standpoint, do these points connect to ground? In the state the switch is drawn, one of them is grounded through the switch, but I am unsure how they are or where they go when the switch is rotated to select AUX.

Example( the state its in now is TAPE, the switch ring shows the left blue arrow grounded through the rings fat tab and the skinny tab, when rotated one click it selects MAG or Ceramic, but then the right blue arrow becomes grounded through the skinny tab, and the fact the the one contact that was just previously on the skinny tab actually extends into the ring, making contact at all times and so grounding the other point poiinted to by the right blue arrow.) When its turned one more click onto Tuner and another for AUX, its like those two points are not connected at all.

Unless they are suposed to be grounded at all times? From a design point I have no idea what they are for so Im at a stand still.

I hope you can follow my post, I have a hard time explaining thoughts at times.

[loudthud]

The left half of V1 is a preamp that is only used when Phono or Tape inputs are used. The proper equalization network is selected by grounding one of the blue indicated wires. When one of the other inputs is selected, the left half of V1 is not used. By routing the signal to the right half of V1, the left half is bypassed and which if any blue line is grounded does not matter.

[changedsoul]

Great, thanks. Ill give that a try. Not sure when though, in the process of moving. Might be a while before I do it and post. But thanks for the info. It will be nice to have this amp working again, if this works and I didnt mess something else up to bad.

[changedsoul]

Loudthud, Do you understand what the SR3a, SR3b, and SR3c switch is used for and how it ties into the Rumble Filter? That section of the amp is disconnected and Im racking my brain trying to figure out this thing was wired up. Looking at the schematic Im having trouble seeing how this switch routes the signal. Checking the switch itself shows it does exactly what its says it will do, I am just unsure how it gets connected into the circuit because the schematic in that part is hard for me to follow.

Any suggestions?

[loudthud]

It looks to me that there are a couple of mistakes in the schematic. I have made corrections in blue.

SR3 is the function switch that selects stereo, stereo reverse and either channel as a mono source. SR3C grounds the unused preamp in mono mode. SR3A selects the input for the top channel, SR3B selects the input for the bottom channel.

The Runble (sic) filter is simply an R/C filter that is bypassed by the slide switch. In a stereo phonograph, the low frequencies that you want to get rid of are out of phase which may explain the connection between the two 750K resistors from each channel. Otherwise, that connection doesn't make any sense. Generally a rumble filter kicks in around 20-40Hz so it shouldn't affect a guitar very much.

[changedsoul]

Awsome. Thanks so much for your input. Your modifications make perfect sense. If this schematic was indeed drawn wrong, Ill touch it up and send it back to the one I got it from.

[changedsoul]

Back again for more advice. I was able to get the amp working ( well sound comming out anyways ), but it sounds sort of nasty. One channel has distorted bass that easy heard at low volumes. Im not sure what to do about it. Its been quite some time since this amp has been working, many years in fact, and Im woundering if maybe this issue is due to possible bad caps? and if so, any advise on where to aquire replacments would be appreciated.

Also, I was measuring some strange high voltages on the plates of the preamp stage, schematic shows 57.5V, but I was getting up near 180V :O. R12 ( 300k resistor is actually a 250K resistor ) although I cant see this small change make such a big change in voltage. The 480K R8 because of the broken switch is no longer connected to anything. Is it possible that this open connection could cause these high voltages?

Voltages at the output stage seem to be close within 10V to the schematic.

Also, I have been reading a bit on biasing amps and notice this has no way to bias the tubes. Because all this is new to me, does this design incorporate a auto bias type config or someting? And if not, could you give some pointers on how to maybe go about biasing the tubes?

Thanks for all the help.

[loudthud]

This amp uses cathode bias for the output tubes like the 5E3. Do a search on here, there are plenty of threads on how to adjust bias. It's not very critical on this amp.

The left half of V1 and V4 is no longer needed for anything so R8 can be removed.

Distortion is likely caused by old filter capacitors or cathode bypass capacitors. Replacement depends on what is there already, for mechanical fit. Looks like C8, C21, C41, C42 are all 20uF. Voltage rating needs to be 350V or higher. There are 25uF caps connected to the output tubes. (Can't read the C numbers.) They should be replaced with 22uF at 50V. Any other electrolytic caps should be replaced on general principle.

Since your profile does not give your location, part sources are only general suggestions if you are in the USA. There are real (industrial) part suppliers where you can generally order parts over the web. Mouser, DigiKey, Allied Radio, and Newark are big names in the US. There are 'Amp building hobby' places like Hoffman, Mojo, Vibro-world, Flip-tops you can find on the net. The prices higher but it's easier to find what you need. Antique Electronic Supply (tubesandmore.com) is another place. Many large cities in the USA still have one place where you can go and find parts. Ask around to anyone who does repairs locally. Then there is ebay. YMMV.