Little Dot Tube Amps: Vacuum Tube Rolling Guide - Page 59

Quote:

Originally Posted by junkimchi 

Has anyone ordered tubes directly from Russia before? I ask because I purchased a set of tubes from a high rated seller on Ebay almost a month ago but still didn't arrive yet (Russia to USA). Googling the issue it seems like there is a customs delay in Russia but just wanted to ask here if anyone has ordered from Russians before. 

In my experience it can take as long as 40 days to get tubes from Russia, Romania, Lithuania, Georgia and similar countries (to the US). So it is likely that there is nothing to worry about. This is "normal".

If you have a tracking number, this can often be a useful site to track packages:

http://www.17track.net/index_en.shtml

Quote:

Originally Posted by junkimchi 

Has anyone ordered tubes directly from Russia before? I ask because I purchased a set of tubes from a high rated seller on Ebay almost a month ago but still didn't arrive yet (Russia to USA). Googling the issue it seems like there is a customs delay in Russia but just wanted to ask here if anyone has ordered from Russians before. 

I've ordered tubes off ebay from Russia twice without any problem. Delivery was slow but the tubes arrived both times and no problems.

Quote:

Originally Posted by junkimchi 

Has anyone ordered tubes directly from Russia before? I ask because I purchased a set of tubes from a high rated seller on Ebay almost a month ago but still didn't arrive yet (Russia to USA). Googling the issue it seems like there is a customs delay in Russia but just wanted to ask here if anyone has ordered from Russians before. 

i ordered 2 pairs of Tesla 6f32v tubes on the 19 of february and they left Moscow today the 12 marsh  so i expect to get them in a week or so.

Wow holy crap it took one month just to get out of Russia? Man it is true what they say about Soviet Russia. 

6N6P-IR for $13 / piece + $ 8.00 for shipping:

http://tubes-store.com/product_info.php?products_id=519

My old total accumulated to $36 for both tubes and shipping because I bought them for + $2 / piece with the original box.

I have to check for the metal of the grid.

I've been silly... Why bother soldering a wire on very-hard-to-solder-on-because-oxidized tube pins or even wrap a wire around to link two pins semi-permanently when you can just... bridge two hole in the socket without soldering anything. Just by dropping a small |__| shaped wire into the holes (stranded copper wire btw, solid core wouldn't work here, it has to be able to ease around the pins) that will make perfect contact between the tube pins, wire, and socket holes over most of the length of the pins, and be flat enough that the tube rests perfectly in its socket barely higher than usual and perfectly vertical.

Voila, so easy a mod even a cave man could do it, and now you have the "schematics" too. That basically makes your amp compatible with all 7BK base tubes like the 6AU6. Nothing permanent, very good mechanical contact (stranded copper stuck between tube pin and socket hole wrapped around a long side of the pin, very hard to get a failed contact). Just chop up two pieces of wire and bring them down slowly in the holes with nose pliers or whatever. Wires need to fit nicely in the holes to avoid bad contact, or worse, contact with other pins. Just insert the tubes on top of that trying to make somewhat sure the wires don't like fly away from the tubes socket that very second, you never know. Only problem is that when you pull the tubes out, the wire has a 50% chance to come out too. Then again, you should probably re-straighten the stranded copper and reinsert the wires properly with every tube change, just in case; hardly very difficult. I used 26 AWG OFC copper wires btw (from a leftover Mogami 2893 quad cable).

Edit: Yeah, definitely take the mod wires out when you change tubes and re-twist the stranded copper back into a straight shape before reinserting the wires in the socket and then the new tubes. When I changed tubes the second time I tried this, the wires only popped out on one side and stayed in on the other, so I only re-twisted the one that popped out: bad idea, both tubes worked fine until I tapped the amp very lightly to check for microphonics and the tube on the wrong side went SHHHHH on me. Pulled tubes and wires back out, put everything back in properly, no problems. Then again, the pins were pretty dirty too -should have cleaned them first- which was probably not helping to make contact with the wire. At least it shows this "mod" won't make your amp explode or anything (you'll just get a floating cathode and lots of noise if the contact isn't perfect).

I tested that "mod" of sorts with "regular" tubes first, and it worked perfectly. 6AK5 jumper setting and do NOT do it with EF92/91 tubes (as pins 6 and 7 are inverted), but on any 7CM base tubes (6CB6, 6DK6...) it works by strapping grid 3 and cathode together (instead of using the EF92 jumper setting to strap grid 2, grid 3 and anode together), while grid 2 is still strapped to the anode. So, it can also be a good mod for those of you that hate changing jumpers with a passion. Note that for EF92/91 tubes, one could also imagine placing a small wire between pins 6 and 7 to strap grid 2, grid 3 and anode together by hand and stay in the 6AK5 jumper setting all the time -if you really hate jumpers that much. Strapping grid 3 to either anode (EF92 jumper) or to cathode (this "mod") might yield sonic differences -note that the shield is usually connected to grid 3 so connecting the shield to either anode or cathode might also make a difference in noise rejection- or might not, feel free to try while you wait for your 6AU6 tubes you know you'll end up ordering ;) .

Now that the amp is "compatible" with 7BK base tubes, and since I just got my 6AU6 in the mail, it's time to test some new tubes!

GE JAN 6AU6WC 1978 matched in box NOS NIB

Tektronix matched in box GE 8425A/6AU6A 1964 NOS NIB

The Tek matched 8425A are currently burning in for the workday. From just testing them after 15 minutes, I can say these are incredibly clean and musical, and natural and detailed with a black black background and possibly the best clarity I think I've ever gotten out of this amp. They have a very similar structure (shield, grids, top, getter flash) compared to the GE 6GX6/6GY6 I have, which is interesting. What is immediately apparent is how awesome the imaging is on these. The singer is right here, smack in the middle of the scene, on the stage or in the studio, singing with all the qualities and inflections of his/her voice, truly impressive! Only the bass could have a bit more punch and slam at this point, but that's probably my headphones more than the tubes. I would be very surprised these don't come out as A+ tubes after burn-in -as I had speculated they would. Try 'em, they're dirt cheap anyway, and their clarity and imaging might be even better than the beloved CV4015 -which is no easy feat.

Hello audiofanboy.Bit puzzled about the EF92   This is a VM RF tube used in radio/wireless receivers the VM means the gain will vary with signal -used to stop overloading of signal rather than a fixed gain Its specially made for that use.   .EF91 is okay

didnt know it was  being used for audio.

Strapping GRID 2 to Grid 3 ? grid 2 usually carries a high voltage like the plate voltage.

If you were using it as a triode then G2 would be strapped to the plate-pin 5 and yes  G3 can be strapped to the cathode pin 2 to pin 6.

6AU6 can be used in audio.                      

Quote:

Originally Posted by duncan1 

Hello audiofanboy.Bit puzzled about the EF92   This is a VM RF tube used in radio/wireless receivers the VM means the gain will vary with signal -used to stop overloading of signal rather than a fixed gain Its specially made for that use.   .EF91 is okay

didnt know it was  being used for audio.

Strapping GRID 2 to Grid 3 ? grid 2 usually carries a high voltage like the plate voltage.

If you were using it as a triode then G2 would be strapped to the plate-pin 5 and yes  G3 can be strapped to the cathode pin 2 to pin 6.

6AU6 can be used in audio.                      

Yes, if anything the EF92 is kind of the odd ball in the natively compatible tubes from looking at the datasheet. But from experience, it works very well and sounds great; gain isn't huge but it has good authority at all volumes imo. EF91 are obviously not really meant for audio and I haven't seen many other examples where they've been used triode strapped in audio circuits; but again they work very well and have very high gain, which I like.

Strapping grid 2 and 3 together and linked to nothing else would be a problem, as grid 3 is not supposed to receive the kind of high voltage grid 2 gets in pentode mode. But triode strapped by linking grids 2 and 3 and anode -which is what the amp natively does with the EF92 jumper setting- it makes sense and just has both grids at the same potential as the anode. Pins 5 and 6 are hardwired together in the amp anyway (which would mean anode and g3 for EF91/92 and wouldn't really work properly without pin 7 = g2 linked as well) and the amp only works with triode-strapped pentode. I just tried strapping g3 to the cathode by hand to triode-strap the tubes differently, and to make the amp compatible with 7BK base tubes like the 6AU6.

The 6AU6, unlike many tubes I tried in the last 2 weeks, is actually good and not rare in audio circuits, and actually has triode curves in its datasheets in the first place; so we can't be that far-off trying it in LD amps. It also has characteristics that make it pretty close to a 6AK5 triode-strapped in terms of gm with a similar circuit biasing, and I can vouch for the fact that it works pretty well right off the bat! In amps wired a bit differently, the 6AU6 is often considered a possible "replacement" for a 6AK5, from what I've read.

Quote:

Originally Posted by Audiofanboy 

I've been silly... Why bother soldering a wire on very-hard-to-solder-on-because-oxidized tube pins or even wrap a wire around to link two pins semi-permanently when you can just... bridge two hole in the socket without soldering anything. Just by dropping a small |__| shaped wire into the holes (stranded copper wire btw, solid core wouldn't work here, it has to be able to ease around the pins) that will make perfect contact between the tube pins, wire, and socket holes over most of the length of the pins, and be flat enough that the tube rests perfectly in its socket barely higher than usual and perfectly vertical.

 

 

 

Voila, so easy a mod even a cave man could do it, and now you have the "schematics" too. That basically makes your amp compatible with all 7BK base tubes like the 6AU6. Nothing permanent, very good mechanical contact (stranded copper stuck between tube pin and socket hole wrapped around a long side of the pin, very hard to get a failed contact). Just chop up two pieces of wire and bring them down slowly in the holes with nose pliers or whatever. Wires need to fit nicely in the holes to avoid bad contact, or worse, contact with other pins. Just insert the tubes on top of that trying to make somewhat sure the wires don't like fly away from the tubes socket that very second, you never know. Only problem is that when you pull the tubes out, the wire has a 50% chance to come out too. Then again, you should probably re-straighten the stranded copper and reinsert the wires properly with every tube change, just in case; hardly very difficult. I used 26 AWG OFC copper wires btw (from a leftover Mogami 2893 quad cable).

 

Edit: Yeah, definitely take the mod wires out when you change tubes and re-twist the stranded copper back into a straight shape before reinserting the wires in the socket and then the new tubes. When I changed tubes the second time I tried this, the wires only popped out on one side and stayed in on the other, so I only re-twisted the one that popped out: bad idea, both tubes worked fine until I tapped the amp very lightly to check for microphonics and the tube on the wrong side went SHHHHH on me. Pulled tubes and wires back out, put everything back in properly, no problems. Then again, the pins were pretty dirty too -should have cleaned them first- which was probably not helping to make contact with the wire. At least it shows this "mod" won't make your amp explode or anything (you'll just get a floating cathode and lots of noise if the contact isn't perfect).

 

I tested that "mod" of sorts with "regular" tubes first, and it worked perfectly. 6AK5 jumper setting and do NOT do it with EF92/91 tubes (as pins 6 and 7 are inverted), but on any 7CM base tubes (6CB6, 6DK6...) it works by strapping grid 3 and cathode together (instead of using the EF92 jumper setting to strap grid 2, grid 3 and anode together), while grid 2 is still strapped to the anode. So, it can also be a good mod for those of you that hate changing jumpers with a passion. Note that for EF92/91 tubes, one could also imagine placing a small wire between pins 6 and 7 to strap grid 2, grid 3 and anode together by hand and stay in the 6AK5 jumper setting all the time -if you really hate jumpers that much. Strapping grid 3 to either anode (EF92 jumper) or to cathode (this "mod") might yield sonic differences -note that the shield is usually connected to grid 3 so connecting the shield to either anode or cathode might also make a difference in noise rejection- or might not, feel free to try while you wait for your 6AU6 tubes you know you'll end up ordering ;) .

 

Now that the amp is "compatible" with 7BK base tubes, and since I just got my 6AU6 in the mail, it's time to test some new tubes!

 

GE JAN 6AU6WC 1978 matched in box NOS NIB

 

 

Tektronix matched in box GE 8425A/6AU6A 1964 NOS NIB

 

 

The Tek matched 8425A are currently burning in for the workday. From just testing them after 15 minutes, I can say these are incredibly clean and musical, and natural and detailed with a black black background and possibly the best clarity I think I've ever gotten out of this amp. They have a very similar structure (shield, grids, top, getter flash) compared to the GE 6GX6/6GY6 I have, which is interesting. What is immediately apparent is how awesome the imaging is on these. The singer is right here, smack in the middle of the scene, on the stage or in the studio, singing with all the qualities and inflections of his/her voice, truly impressive! Only the bass could have a bit more punch and slam at this point, but that's probably my headphones more than the tubes. I would be very surprised these don't come out as A+ tubes after burn-in -as I had speculated they would. Try 'em, they're dirt cheap anyway, and their clarity and imaging might be even better than the beloved CV4015 -which is no easy feat.

Are these 8425a compatible with a mk3 or mk4 or both ? Thank you

Quote:

Originally Posted by MIKELAP 

Are these 8425a compatible with a mk3 or mk4 or both ? Thank you

Both and probably the MK II and I+, basically all the LD amps made for 6AK5 type tubes. Again though, not natively.

There are no differences in design between the MK III and IV as far as I can tell from the pictures of the circuit boards on the LD website, just different and better/larger components, but similar schematics.

Quote:

Originally Posted by Audiofanboy 

I've been silly... Why bother soldering a wire on very-hard-to-solder-on-because-oxidized tube pins or even wrap a wire around to link two pins semi-permanently when you can just... bridge two hole in the socket without soldering anything. Just by dropping a small |__| shaped wire into the holes (stranded copper wire btw, solid core wouldn't work here, it has to be able to ease around the pins) that will make perfect contact between the tube pins, wire, and socket holes over most of the length of the pins, and be flat enough that the tube rests perfectly in its socket barely higher than usual and perfectly vertical.

 

What a great idea! And while I don't have any suitably small stranded wire, the 30 gauge solid core wrapping wire I have is also working fine for this purpose.

And I agree about the sound of these tubes. I have plain-Jane used GE 6AU6s and I am amazed at what I am hearing. And the 6136s and 8425As I have coming might be even better? :)

Oh, MIKELAP, I can attest to the fact that all this is working fine in a 1+.

Quote:

Originally Posted by gibosi 

 

What a great idea! And while I don't have any suitably small stranded wire, the 30 gauge solid core wrapping wire I have is also working fine for this purpose.

 

And I agree about the sound of these tubes. I have plain-Jane used GE 6AU6s and I am amazed at what I am hearing. And the 6136s and 8425As I have coming might be even better? :)

 

Oh, MIKELAP, I can attest to the fact that all this is working fine in a 1+

Thanks for  your imput it is appreciated . actually i did not realize you had a mk4 tough it was a 3 at the same time i was wondering if i ever bought a mk4 if my tubes would work with this amp apparently they do but does the mk4 have additionnal tubes it can use or it is the same tubes as mk3 While im at it is there a big sound difference between the mk3 and mk4se. Thanks again.

Quote:

Originally Posted by MIKELAP 

Thanks for  your imput it is appreciated . actually i did not realize you had a mk4 tough it was a 3 at the same time i was wondering if i ever bought a mk4 if my tubes would work with this amp apparently they do but does the mk4 have additionnal tubes it can use or it is the same tubes as mk3 While im at it is there a big sound difference between the mk3 and mk4se. Thanks again.

The MK IV ( and IV SE) was designed from the get go to work with 6H30P tubes, whereas the (original) MK III was not. That is no longer true as the MK III works quite well with 6H30P tubes now. So, as far as compatibility goes, MK III & IV are the same. I've never had the chance to listen to a MK III unit but I assume there is a difference between the MK III & IV, and in turn between the MK IV & IV SE (if you look at the circuit board pictures on the LD website, if anything just at the caps which have a different size on the IV). That added to other hardware must surely amount to some degree of difference, like maybe the last 30% or something, but surely the tubes you use would account for the first 70% or so of performance. All I know is that I get great performance out of my MK IV SE with maxed out power tubes and delicious driver tubes ;) .

Hi Folks,

Here is an answer to a question that I am sure you all have: What is the difference between 6CB6 and 6CB6A vacuum tubes? Since I like Tung Sol tubes I ordered a set and received 6CB6 tubes from 1957. After 20 hours burn in they sound delicious.(Hopefully, a comparison of these tubes with the EF92 Mullards will follow).

However, should I have ordered GE 6CB6A tubes instead?

Now, here is the answer about the difference:

"In the early days all TVs and most radios had power transformers. The tube heaters were all wired in parallel and fed 6.3 volts. As with everything, cost reduction eventually chased the power transformer out of TV sets. The tube heaters got wired in series, running directly from the line voltage. Tube heaters have a positive temperature coefficient. Their resistance is low and increases as it warms up. When they are all in series, they better all warm up in the same manner, or the fastest tube will get all of the voltage and light up like a flashbulb. It was decreed that the warm up time will be controlled and specified to be 11 seconds. The conditions for measuring the 11 seconds was specified and each tube got its revision letter upreved when the change was made. This is usually clearly specified in the data sheet.

The 1961 GE datasheet that is on my hard drive does specify an 11 second warm up time, it is not spelled out like it is in other books.

I have a 1966 edition of the RCA Receiving Tube Manual that I bought new at Lafayette Radio Electronics in 1966 for $1.25. It still has the LRE sticker on it. Here it states that "types 3CB6, 4CB6, and 6CB6 are identical with type 6CB6A except for heater ratings". In the specs all are listed at 11 seconds except for the 6CB6.

In my 1963 RCA book it states "Type 6CB6A has a controlled heater warm-up time for use in television receivers employing series-connected heater strings."

The A has a slow warmup feature for the heater so it doesn't blow as quickly. The 'A' types are going to be much more common since so many of these were required by TV set manufacturers. "   So far the quotes.

Can anybody tell me if there would be a difference in the sound between 6CB6 and 6CB6A tubes? Are 3CB6 and 4CB6 tubes good alternatives for LD MKIII. Any impressions about sound quality?