As part of the 7551's I also got 1 pc of a RCA metal can 6L6 tube for $5. Based on reading they are supposedly not a good listening tube but I am assuming they could be used as test tubes to prove out the rest of the chassis. These tubes are very reasonable on EBAY. Also interested in anyone offering up any inexpensive 6L6 tubes.
Obviously not interested in having a lot of money in tubes until some confidence in gained in the chassis.
I hope you realize that you can not use anyother tube in the 7355 sockets unless you rewire the sockets. Also installing other power tubes into the circuit will upset the use of the bias voltage from the power tubes to power the filaments of your preamp tubes. The more you make changes with the original circuit the greater the likelihood of making mistakes and having to troubleshoot problems. Unless you are adept at building tube amps you should try to keep everything stock until it is working right. Otherwise, you could be chasing your tail. regards, 808
DAK808 based on reading the 6L6 are almost drop in other than the socket rewire. No Bias change required is what I thought I read.
Hi jlangholzj,
Actually, my mistake and please excuse the error. I was just dealing with a 6BQ5 bunch of questions and for some reason that continued over to here.
The heater requirements of a tube like the EL34/6CA7 exclude it from use, and a 6L6 is a touch higher, but probably fine in this amplifier after the sockets are rewired. If I owned this amplifier, I would definitely change it over to use 6L6GC outputs, or 7591A's if I was confident that supply would always be available.
-Chris
Actually, my mistake and please excuse the error. I was just dealing with a 6BQ5 bunch of questions and for some reason that continued over to here.
The heater requirements of a tube like the EL34/6CA7 exclude it from use, and a 6L6 is a touch higher, but probably fine in this amplifier after the sockets are rewired. If I owned this amplifier, I would definitely change it over to use 6L6GC outputs, or 7591A's if I was confident that supply would always be available.
-Chris
They're close enough to each other that it should be, yes. You're going to have to make sure once you do replace them and re-wire however that you've got everything happy.
No probs, was just trying to clear anything up in event of confusion! For tubes with a higher current draw it's not too much of a hassle to add in a Hammond 166 series or equivalent to power the power stage filaments and then also pick up the extra pre-amps with said transformer. This is the route I'll be heading with mine but you are correct, in stock form you're limited in options.
IMO, why go through all the bother of rewiring the sockets and such. The amp was designed around a certain tube and all you need to do is acquire some and move on. You already have your work cut out for you in rebuilding the power supply changing coupling caps , etc, etc.
Worrying about tube availability is somewhat premature at this stage. You don't even know if the amp is fully functional.
On top of that you are going to rewire all the power tube sockets? Piling up levels of complexity only increases the chances of errors.
You will still need a good set of power tubes anyway. So,It just seems like a lot of hassle to me to change the power tubes. I have converted 7868 to 7591 and 6bq5 to 6v6. You are looking at from 2 to 4 hours of work that you might want to spend on a different area. Add more time if you break off a solder lug while removing components and need to buy a new socket. Add more time if you make a mistake wiring or soldering and need to troubleshoot. Add more time and dollars if while troubleshooting your mistake or making a wiring error you fry the output transformer or power transformer. Good luck, 808
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Hi DAK808,
I know your heart is in the right place, but I completely understand why Bob would want to change to a more available (less costly) tube type. In fact, the rewiring of the tube socket is minor, only two changes. Once he has one socket done and checked, he can copy those to the remaining three. If he runs into trouble, we are here.
Your concern over breaking the lug off a socket is justified. I usually use a solder sucker to remove most of the solder, then I cut the leads in the loop part of the wire. Restrip the wire, or clean the component lead and they can be connected to their new location. Snapping lugs off is rare if this procedure is followed. The remainder of the wire or lead on the old location is easily cut some more, or heated until the wire end moves. Keep moving it as it cools. Often the wire bit will fall out doing this, if not, it can be cut again so that it does literally fall off.
-Chris
I know your heart is in the right place, but I completely understand why Bob would want to change to a more available (less costly) tube type. In fact, the rewiring of the tube socket is minor, only two changes. Once he has one socket done and checked, he can copy those to the remaining three. If he runs into trouble, we are here.
Your concern over breaking the lug off a socket is justified. I usually use a solder sucker to remove most of the solder, then I cut the leads in the loop part of the wire. Restrip the wire, or clean the component lead and they can be connected to their new location. Snapping lugs off is rare if this procedure is followed. The remainder of the wire or lead on the old location is easily cut some more, or heated until the wire end moves. Keep moving it as it cools. Often the wire bit will fall out doing this, if not, it can be cut again so that it does literally fall off.
-Chris
I am looking at a trio of Baldwin organ 6L6 GC tubes. I was going to use the trio with the one RCA 6L6 metal can tubes that I have temporarily to test out the chassis. Currently I can get the trio for $42. These have been tested at 36,36,34 or 4500,4500,4250.
If these prove out to be good tubes would I be able to find a 4th tube to match the trio?
If these prove out to be good tubes would I be able to find a 4th tube to match the trio?
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Not how that works. If you're going to spend forty some odd dollards on a trio, might as well spend an additional $10 on the 4th one. Besides that there's several matched quads (in the US mind you) on the fleaybay currently that are cheaper than what you're worried about for the trio!
I have not heard many good things about the inexpensive...supposedly matched quad sets. Reading up on the Baldwin tubes they sound like a quality vintage tube.
A lot of those inexpensive quads are not 6L6GC tubes rated at 30Watts.
If you know where a quad set of 6L6GC tubes are available on Ebay for less than $45 let me know.
The 6L6GCR is a 5881A tube rated at 23W.
A lot of those inexpensive quads are not 6L6GC tubes rated at 30Watts.
If you know where a quad set of 6L6GC tubes are available on Ebay for less than $45 let me know.
The 6L6GCR is a 5881A tube rated at 23W.
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So you're splitting hairs on "inexpensive supposedly matched sets" and yet you want to buy a trio so you can put your other 6l6 with them......and then maybe find another matching tube later for them? 🙄
Hi Bob,
I have ordered and used matched quads of 6CA7EH, 6L6EH and 7591EH and have never been dissatisfied. Notice how these excellent tubes are Canadian - EH. 🙂
If I were you, I would lay down some money for the matched quad (new) and be done with it. Buying "pulls" isn't a good plan - having gone that route may years ago. False economy for sure!
A 5881A is not any form of 6L6, 6L6G, 6L6GB or 6L6GC! It is a completely different tube all on it's own. As you noted, it has a 25 watt plate. The 6L6 is a 19 watt plate and the others are 30 watt plates. A 7581A is a 6L6GC with a 35 watt plate, sharing the plate characteristics with the 6L6 family. A trait not shared with the 5881. This is an error that has existed for decades. Probably from the musicians and poorly trained technicians. They might like how that tube sounds in guitar amps, but for domestic audio reproduction, the 5881 will deliver less power (or have a greatly shortened life) and operate with higher distortion.
Where on earth did you hear of a"6L6GCR". That type number doesn't even make any sense. Only someone who doesn't know anything about tubes would put a part number like that together! Never mind that the 5881A is rated at 25 watts as it normally sits. Good luck with that!
-Chris
I have ordered and used matched quads of 6CA7EH, 6L6EH and 7591EH and have never been dissatisfied. Notice how these excellent tubes are Canadian - EH. 🙂
If I were you, I would lay down some money for the matched quad (new) and be done with it. Buying "pulls" isn't a good plan - having gone that route may years ago. False economy for sure!
A 5881A is not any form of 6L6, 6L6G, 6L6GB or 6L6GC! It is a completely different tube all on it's own. As you noted, it has a 25 watt plate. The 6L6 is a 19 watt plate and the others are 30 watt plates. A 7581A is a 6L6GC with a 35 watt plate, sharing the plate characteristics with the 6L6 family. A trait not shared with the 5881. This is an error that has existed for decades. Probably from the musicians and poorly trained technicians. They might like how that tube sounds in guitar amps, but for domestic audio reproduction, the 5881 will deliver less power (or have a greatly shortened life) and operate with higher distortion.
Where on earth did you hear of a"6L6GCR". That type number doesn't even make any sense. Only someone who doesn't know anything about tubes would put a part number like that together! Never mind that the 5881A is rated at 25 watts as it normally sits. Good luck with that!
-Chris
Okay got out a 6L6-GC spec sheet and now am aware that there is more wiring changes required than what was required for the 7591XYZ tube substitute.
To rewire the output sockets to accept the 6L6 tube will require removing all wires/components on pin #4 and reposition on an insulated lug. What is the extent of additional rewire?
Pin 2 and 7 heater will remain the same.
Pin 3 plate will remain the same.
Pin 4 will have no connections.
Pin 6 Grid #1 gets moved to Pin 5?
Pin 8 Grid #2 (screen) gets moved to Pin 4
Pin 5 Cathode gets moved to Pin 8?
Help!
To rewire the output sockets to accept the 6L6 tube will require removing all wires/components on pin #4 and reposition on an insulated lug. What is the extent of additional rewire?
Pin 2 and 7 heater will remain the same.
Pin 3 plate will remain the same.
Pin 4 will have no connections.
Pin 6 Grid #1 gets moved to Pin 5?
Pin 8 Grid #2 (screen) gets moved to Pin 4
Pin 5 Cathode gets moved to Pin 8?
Help!
I quick glanced at the two and I think these should be the pin switches but re-verify first.
7355---------6L6
1-------------1--------not used
2-------------2--------heater_1
3-------------3--------plate/anode
4-------------6--------not used
5-------------8--------cathode
6-------------5--------grid
7-------------7--------heater_2
8-------------4--------screen
Although if you're struggling with matching pin-out I'd highly suggest you start reading some literature and so-forth first so you understand where all this goes and why.
7355---------6L6
1-------------1--------not used
2-------------2--------heater_1
3-------------3--------plate/anode
4-------------6--------not used
5-------------8--------cathode
6-------------5--------grid
7-------------7--------heater_2
8-------------4--------screen
Although if you're struggling with matching pin-out I'd highly suggest you start reading some literature and so-forth first so you understand where all this goes and why.
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The more I look the more I lean to the 6L6GC tubes. I am not so concerned about the wiring.
I have a question concerning the biasing of the 6L6GC tube. The spec sheet shows a -37V for a push pull amplifier application. The HK A50K looks like it was biasing the 7355 tubes with 35V. If the rewiring is done and a set of Electro Harmonix 6L6GC tubes are installed there should be no need to change the bias?
I have a question concerning the biasing of the 6L6GC tube. The spec sheet shows a -37V for a push pull amplifier application. The HK A50K looks like it was biasing the 7355 tubes with 35V. If the rewiring is done and a set of Electro Harmonix 6L6GC tubes are installed there should be no need to change the bias?
The -37 is NOT a "spec". Here's what I posted in an FAQ on another forum:
Keep one last thing in mind - often you see a voltage "spec" on a schematic at the control grid of a tube. DO NOT TRY TO SET THE VOLTAGE AT THE GRID TO THAT SPEC! The negative voltage at the tube grid should be whatever value is required to set the tube's cathode current to spec. If the tube is passing the correct amount of current and the voltage at the grid is -20 volts then the hell with the "spec" of -22.5 volts or whatever it is. Making the grid voltage more negative with respect to the tube's cathode is the equivalent of stepping down harder on a garden hose to restrict the flow of water. And if you let up the pressure on the hose and more water flows - that's the same thing that happens if the tube grid is less negative!!
Whatever that bias voltage ends up at will depend on the tubes as well. so consider the -37 as a guide, not a spec. What makes it more difficult in this case is the infuriatingly CHEAP bias setup the A50K uses.
Hi Jim,
That's about as effectively explained as I had ever seen. I was about to say pretty much the same thing you did, less the bias supply comment. I've seen to many lousy setups for that to even register these days.
Hi Bob,
That's the real bit of knowledge to remember from this. Tube bias voltage is incidental as it is whatever the tube needs to run at the proper current. You can take note of it once you have it set up for interest's sake. Nothing to worry about unless you are way off, then you have a problem.
-Chris
That's about as effectively explained as I had ever seen. I was about to say pretty much the same thing you did, less the bias supply comment. I've seen to many lousy setups for that to even register these days.
Hi Bob,
That's the real bit of knowledge to remember from this. Tube bias voltage is incidental as it is whatever the tube needs to run at the proper current. You can take note of it once you have it set up for interest's sake. Nothing to worry about unless you are way off, then you have a problem.
-Chris
And I remember it being on thing that I really struggled with when I first was looking at getting into tubes and so-forth. Also OP you hear it from the HK horse's mouth, per-say. If you get a chance go browse through Jim's work on the Citation and HK chassis along with about anything else on the internet he's posted in various forums. I remember finding lots of good information when I started with my A500.