Hopeful to find help with a vintage Japanese tube amp

[fastmerc]

Hello folks,
I recently picked up a 60's Recco tube amp. I didn't expect much but I actually like it. It does seem to have an issue. There is an onboard temelo effect and that doesn't seem to work. It may have been bypassed by someone prior. I'm a newbie to amp repair so any help and advice is appreciated. When you turn the trem control knobs there is a change in sound but it is more of a tone change not a trem effect change. Here are some pics which are a tab blurry but hopefully recognizable enough. The two parts circled in red may have been replacements or additions, and the part in blue may be bad just by looking at the colors.

[JHow]

Do you have a switch for your tremelo?

[fastmerc]

I do not have the true switch but I used a regular plug in jack and shorted the wires just like a switch would do.

[JHow]

Can you tell people other information like what tubes it uses, is there a model number on it, do you have a schematic?

[fastmerc]

The model is 505, I don't have a schematic and haven't been able to find one online. The tubes are 2-12ax7 and the other 2 I can only see the markings 5f

[Gtr_tech]

The output tube sockets look like 7 pin, so I'd say 6AQ5 outputs.

[fastmerc]

That is the same info that I just found in a review somewhere about the output tubes.

[JHow]

I was wondering if it might not be 6CA5. Different pin-out but still seven-pins and about half the power than 6AQ5.

I think you are probably on your own for drawing up a schemo.

As far as the tremelo goes, if you know which 12ax7 is doing the "tremming" you could swap with a known good one and see if that makes a difference. Another thought on the tremelo is that some designs will have a smallish 8-10uF or so electrolytic cap in there that could simply be old at this point.

[fastmerc]

Well im really newb at this so I can try and draw something out. How would I tell the difference between a 6aq5 and a 6ca5. As far as the caps there are 2 large ones on the underside of the unit. 1 bigger than the other.

[JHow]

Darn. I tried to reply, but I guess there was some confusion on the server and my reply doesn't seem to have posted.

To your question about the different tubes. It is academic to some extent, since the ones you have work. Since the tremelo is the thing that isn't working, I would start with that. The short answer to your question is that by looking at the voltages and pins one could make a reasonable guess as to what the tubes are. Since the power tubes work right now, I wouldn't fuss with them.

One idea I had was you can sometimes read faint tube markings if you "fog" the tube with your breath. You might give that a try - it will cost you nothing.

Regarding the tremelo, did you try swapping in a known good 12ax7?

To answer your question about the big filter caps, they are multi-section (several caps in a single can with common neg.) They should be marked on the outside what the values are. They can be three or four in one can. You might see something like 40-40-30-20, which corresponds to 40uF, 40uf, 30uF, 20uf, etc. They will usually have "codes" for the lugs, so 40uf might have a little triangle, or square, or half-moon marking its positive lug.

However, if your amp plays and does not have large amounts of hum we can assume that these caps are doing their job and don't need to be messed with. Please note, these caps can store a lethal charge, even when the amp is unplugged, and they can store this charge for a very long time. There is a simple procedure to discharge them, which you should research if you don't know it.

Which brings up the question: Do you know how to safely measure voltages on a live amp? Do you know how to discharge caps? Do you have a meter? If you looked at schematics similar to your amp can you follow them and supply the missing info. for you amp? Sorry if these questions are duck soup, but it is hard to diagnose an amp looking at pictures.

[52 Bill]

The pinouts for the 6AQ5 and the 6CA5 are entirely different and are not interchangeable. If you search online there will be many sites that offer basic tube manual information. This info will show you what pin is connected to the filament, the grid, the plate, etc. With this information you can guesstimate as to what the tube type is.

[fastmerc]

Necroing this old thread. With the recent success of working on the Peavey, I dug this one out to take another swing at it. To answer some questions that were asked, Yes I tried putting in 2 new 12ax7s and that did not fix the tremolo issue. I will be taking good pictures soon and posting them. This one is much simpler in design than even the Peavey was so Im hopeful with the assistance of folks here I can get this one up and running.

[fastmerc]

The first pic here has 2 items circled....the first is a 200ohm resistor (I believe anyway). It measures out ok but is obviously discolored and it gets super hot when the amp is on. The 2cd is another resistor marked 5w1K, which I take to mean a 5 watt 1k ohm and it is bad for sure. For a replacement should I go back with a similar one or go with one of the white ceramic kind? I have tested all the resistors on the preamp circuit board and they all seem to be ok...there were about 4 that didn't measure up at first, but when I pulled them from the board they ended up being ok. I tested as many Caps as my meter would allow. There are several caps that are marked .02uf they measured .035....but they all measured that and that was in circuit. Also the amp seems to crackle alot while on. Anyway on to the pics:

[fastmerc]

and the last pic

[52 Bill]

The 200 ohm resistor is the cathode resistor for the two output tubes. It is paralleled by the 50uF cap that is next to it on the tag board. They will get hot, but you may have a bad output tube causing it to get hotter than normal.

The 1K resistor is the first resistor in the power supply line. It feeds the screen grids of the two power tubes. If it need to be replaced, a more typical white ceramic wirewound will be fine there.