[aesthetik]

Quote:

More Tube Rolling: I decided to go out on my own this time and try some tubes that interested me. One of which others have tried on a "stock" 708B, but never on a modded one. This is the NOS GE JAN 5670. This one in particular is from 1968. The reason no one liked this tube is because it created too much hum or noise into the signal. Again, this was with all stock 708B amps. However, I put this 5670 tube in my amp and it is totally, completely and 100% dead silent, even at max volume and everywhere in between. I also want to try some different output tubes as well. I've heard that the Amperex-Japan 6922/E88CC tubes were actually made by Matsu****a, which are very VERY good tubes. Here they are... Out with the NOS RCA 6DJ8 output tubes and NOS Russian Military 6N3P EB driver tube... And in with the new (NOS) GE JAN 5670 & Amperex-Japan 6922/E88CC tubes. As for the sound, I feel that they have a better "grip" on the music than the UK made RCA 6DJ8's and Russian 6N3P EB that drarthurwells recommended. They DO have a wider soundstage, the dynamics are more dynamic, bass seems to be a bit tighter and definately deeper, the midrange is full and natural, and the treble is still smooth, crisp and extended. And a close-up of them in action.

Hi all,

Hope no 1 minds the bump.

I just got mine yesterday, paid about 110€ all together with shipping. May be a bit expensive for an American but in Europe it's a pretty good price.

OK, I'm just looking for a stupidly simple clarification on the tubes.

Mine has 5 tubes. 4 that are the same size as only what I can call a 12ax7 (I only know that from playing guitars for 22 years.)

Two are in a row of three and say 6n. (circuit board VE2 and VE3)

1 of that row of 3 says nothing on the tube. (VE1)

1 is the 1 in the view window. (also no number printed on it)

A 5th is a different type all together and is much thinner.(6z4)

Could someone clarify which to change and with what?

I know there is some difference preference sound wise, and I'd be for hearing all the different likes and dislikes, but I need it dumbed down to which and how many are to be changed and which can stay.

Thanks in advance.

 

[aesthetik]

ok, think I'm going to answer my own question here just want confirmation.

Found a clearer post in the thread.

Quote:

The front tube of the three tubes inside the amp (not the front window, that's just for show as you said) is the gain tube and should have the biggest influence on the sound. For this you can use: For the 6N3 ------> 6N3P, 5670, ECC42, 2C51 and 396A The two behind that are the left and right channels: For the 6N11 -----> 6DJ8, 6922, E88CC, ECC88, PCC88, E188CC, 7308, 7DJ8 and 6N23

Just want to be sure....

1. Window tube is just a dummy?

2. Smaller tube is rectifier, no need to change for sound quality?

3. Front tube can be replaced with 6N3P, 5670, ECC42, 2C51 and 396A?

4. and back 2 tubes with 6DJ8, 6922, E88CC, ECC88, PCC88, E188CC, 7308, 7DJ8 and 6N23?

5.wrap up, only the 3 in a row tubes need to be changed for sound quality?

 

[dcheming]

Sorry I missed this.

1. The tube in the window is indeed only for show and can be removed completely without affecting the functionality of the amp.

2. The taller, thinner tube is indeed the 6Z4 rectifier. I believe there are only a few different brand 6Z4 tubes made and they're all the same basic quality level. So even if you did want to roll this I don't think it'd be worth it. Spend your money on the other three tubes which will make a difference.

3 & 4. I've only replaced the two 6N11's with some 6DJ8's so I can't directly comment on this. You have to be a bit careful because even if the pinout is the same between two different types the heaters might draw too much current for the power transformer to handle or the operating points might be different enough between them to cause issues. So some tubes on that list are closer substitutes than others. One of the sources that I use for checking this is this site.

5. Pretty much. This is where I'd focus on to get the most bang for your buck.

Quote:

Originally Posted by aesthetik /img/forum/go_quote.gif ok, think I'm going to answer my own question here just want confirmation. Found a clearer post in the thread. Just want to be sure.... 1. Window tube is just a dummy? 2. Smaller tube is rectifier, no need to change for sound quality? 3. Front tube can be replaced with 6N3P, 5670, ECC42, 2C51 and 396A? 4. and back 2 tubes with 6DJ8, 6922, E88CC, ECC88, PCC88, E188CC, 7308, 7DJ8 and 6N23? 5.wrap up, only the 3 in a row tubes need to be changed for sound quality?

 

[Chops]

How about this... It's been almost exactly a year since I was last on here!

Shortly after my last post here, my computer got slammed hard with a virus, thus taking out my hard drives and my website that I was hosting. Then shortly after that, I decided it was time for a new job. So since then I have been working my tail off with this new job, am a manager in training and have much better pay and benefits.

Also since my last post, my amp has remained the same and has been sitting for the entire time unused. I also sold my AKG K701's! However, tonight I just decided to fire up the ole' 708B and use it with my trusty Sony MDR-7506's. Those 1500uF caps certainly did take away a good chunk of the bass, so if I keep listening to this amp again, I'm going to put those Mundorf caps back in.

In the meantime when not working, I have been focusing all of my energy, time and mostly money into my 2-channel/7.1 home theater system. I have been spending some mad cash on this system, especailly lately. As it is, I have a pair of speakers on order right now waiting to be shipped.

Anyway, I have a new website up. Actually it's more of a gallery than anything, but I'm no longer hosting it on my PC like before. Pics load much faster and the layout looks great. It's: Zenfolio | CharlesT Photography

I think I'm going to be sticking around again for a while. There's some really decent people here who I lost complete contact with over the past year. So there you have it!

Since I can't post without leaving at least one pic, here's a pic of the speakers I'm getting. They are the Onix X-Statik from AV123.com. They're a hybrid sealed/open baffle setup. I'll give more info in my next post!

 

[Chops]

Well I just finished uploading the 708B pictures to my site then reinserting them into this thread in all my various posts. I also reinstalled the Mundorf output caps into my amp. They really are much better sounding than the 1500uF caps I had in there.

I hope to keep this thread going as it is chalk full of great information. It's simply amazing how far you can take one of these amps. A true sleeper!

 

[Chops]

Another update...

I've only had this amp back up and running for a few days now and I've already done another mod to it... Kinda.

I had a couple of Solen 51uF caps sitting around that I used in a loudspeaker project a short while back. They were used to absorb any turn-on thump from an amp to a pair of compression horn drivers (as they were connected directly to the amp and were crossed over via a digital x-over). However, there was never any turn-on thump, but just in case there was.

I decided to try them as bypass caps on my Mundorf's. I just have them connected will alligator clips for the moment until I can figure out how to install them. I think it may be time to think about getting a new chassis to transfer everything over to as I'm growing out of the current 708B chassis.

Initail impressions are that these Solen caps remove a bit of grain from the upper midrange and treble and also thicken up the bass just a tad bit. This is more the way a tube amp is supposed to sound. I also notice that there's more inner detail to be had now as well a little more spaciousness. BTW, these Solen 51uF caps are even larger than the Mundorf's!!

What is it with me and huge caps?!?!

However, this is also listening through my Sony MDR-7506's which can tend to be a bit bright and a shade thin in the midrange, dare I say a little harsh even. Maybe I should look into getting another pair of better cans again, but nothing overly expensive this time around, and something easier to power as well.

Anyway, as usual, I have pictures! Imagine that! LOL So here we go...

 

[jrhill]

Hello All,
This is my first post here and in the middle of modding a friends 708B and thought I'd add a bit of measured info for others to see.

In Oz, we have 240v, 50Hz and this gives high volts (B+) of +215 on pcb, heaters of 7.3v ac, and the current thru the input valve (6N3, etc) is about 2.7mA, and 10.5 mA thru the 6N11s.

Reducing the B+ to 195v via p/s resistors (1k2 for the 1k0 and 2k2 for the 2k0) and also added 1R0 in heater line=6.3vac.

Some resistor changes are 34.5kR for 33k, 230R for 220R (on 6N11) and the rest of the resistors were simply changed to good quality Beyschlag M/oxides.

All the other upgrades completed too, including replacing the o/p wire and the crappy socket.

Perhaps this may be useful.

... jh

 

[glitch39]

@jrHill - this is useful

A lot of tube amp enthusiasts also have a tendency to roll tubes and tinker a bit. The 708B is very simple, has lots of space to work with and parts are relatively reasonable. By that I mean the knowledge learned from the tweaking is invaluable.

My 708B has been a reliable component for a while now.

 

[Chops]

Quote:

Originally Posted by jrhill /img/forum/go_quote.gif Hello All, This is my first post here and in the middle of modding a friends 708B and thought I'd add a bit of measured info for others to see. In Oz, we have 240v, 50Hz and this gives high volts (B+) of +215 on pcb, heaters of 7.3v ac, and the current thru the input valve (6N3, etc) is about 2.7mA, and 10.5 mA thru the 6N11s. Reducing the B+ to 195v via p/s resistors (1k2 for the 1k0 and 2k2 for the 2k0) and also added 1R0 in heater line=6.3vac. Some resistor changes are 34.5kR for 33k, 230R for 220R (on 6N11) and the rest of the resistors were simply changed to good quality Beyschlag M/oxides. All the other upgrades completed too, including replacing the o/p wire and the crappy socket. Perhaps this may be useful. ... jh

I think most of us hear in the states have done this mod as well considering we are 120V/60Hz, 10V higher than the 708B p/s. As for the rest of the resistors, every single one in my amp has been replaced.

 

[jrhill]

Yeah, following on from your work and others in this thread is like getting first hand instructions and made the upgrade quite straightforward - many thanks to you and all the others for writing up your work ... jh

 

[Chops]

Quote:

Originally Posted by jrhill /img/forum/go_quote.gif Yeah, following on from your work and others in this thread is like getting first hand instructions and made the upgrade quite straightforward - many thanks to you and all the others for writing up your work ... jh

I'm glad to hear that we were able to help in some way. It is much easier to perform mods to something when you have good instructions and plenty of good pictures to go by. I certainly wouldn't have been able to do any of this stuff without Derek's (dcheming) help throughout it all. You ask that guy a question and he tells you everything you need to know in great detail!

As if you couldn't tell, I'm trying to bring this thread back to life. I am getting back into headphones again and just bought a pair of HD600's (as seen in my Sig below). In fact, years ago, I started with a pair of Sennheiser HD545's. A couple months later I moved up to HD580's, then a couple months later the HD600's. That was over 5 years ago! Then a little over a year ago, I got into headphones again and bought the AKG K701's. And now I'm going back to the HD600's. Go figure... I think I might be mental! LOL

 

[Cloth Ears]

Quote:

Originally Posted by jrhill /img/forum/go_quote.gif Hello All, This is my first post here and in the middle of modding a friends 708B and thought I'd add a bit of measured info for others to see.

Hi Head-fi'ers,

mine is the 708B that James is modding (also my first post, over from DIYaudio & AudioKarma). I'm going along tomorrow to have a listen and discuss, so I'll take the camera and hopefully have a couple of shots to document the progress.

 

[Chops]

Quote:

Originally Posted by Cloth Ears /img/forum/go_quote.gif Hi Head-fi'ers, mine is the 708B that James is modding (also my first post, over from DIYaudio & AudioKarma). I'm going along tomorrow to have a listen and discuss, so I'll take the camera and hopefully have a couple of shots to document the progress.

Welcome Cloth Ears! I knew that name sounded familair. I know you from DIY Audio. Looking forward to seeing some pics and hearing what you have to say about the new mods/sound.


Quote:

Originally Posted by Chops /img/forum/go_quote.gif Maybe I should look into getting another pair of better cans again, but nothing overly expensive this time around, and something easier to power as well.

Well, so much for keeping within my new headphone requirements. I just ordered a pair of HD600's yesterday! LOL

 

[Cloth Ears]

Quote:

Originally Posted by Chops /img/forum/go_quote.gif Well, so much for keeping within my new headphone requirements. I just ordered a pair of HD600's yesterday! LOL

Ain't that always the way...?

I read somewhere on this site (before I'd become a real person and signed up) that someone wasn't sure what the voltage to the audio board should be. So I asked at the source, and received this response:
Quote:

Originally Posted by xshn Dear sir: 708 transformer output alternating voltage is 215V, arrives at the enlargement part d C operation voltage should about 190V.

 

[Chops]

Quote:

Originally Posted by Cloth Ears /img/forum/go_quote.gif Ain't that always the way...? I read somewhere on this site (before I'd become a real person and signed up) that someone wasn't sure what the voltage to the audio board should be. So I asked at the source, and received this response:

Yeah, you'll have to insert a resistor in the heater curcuit to lower the voltage. Mine was 7.58v and after I installed a 1 ohm - 10 watt resistor, it dropped down to 6.63v. That's much better and much closer to the 6.30v that it should be.