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Replacing the driver tubes in an old Luxman A3600 amplifier


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After finishing a thread on receiving my first tube amplifier and bringing the circuit back into line with Luxman specs, I am now turning my attention to replacing the old tubes in it. The Luxman A3600 has three stages and tubes:

1. 6AQ8 x 1

2. 6240G x 2

3. 8045G x 4

 

Here is some marketing spin from Luxman (translated into English from Japanese) on the 6AQ8 (see https://stereonomono.blogspot.com/2011/07/luxman-mq-3600.html):

 

"At the first stage, we adopted the 6AQ8, which offers low inner resistance and large amplification, thus keeping impedance at the plate circuit quite low and preventing deterioration at the high frequency characteristic. Also the phase compensation circuit is banished which tends to deteriorate distortion in high frequency range. With meticulous care, we delved into the inherent characteristics through all the amplifying stages to realize excellent distortion characteristic from the low frequency up to the high frequency range by applying only 16 dB amount of major negative feedback. Of course high stability is assured."

 

So for the first stage, I took XLR8R's advice and bought a Philips Miniwatt tu1 ECC85 tube that tested strong (77/72 = 92%/86%) and it has definitely improved clarity. The ECC85 is the same tube as the 6AQ8, or it has the same characteristics. 

 

Here is Luxman again on the second stage:

 

"At the driver circuit of so-called orthodox "Mullard" type is placed, which provides large output voltage and gain in quite stable condition. Here, the 6240G is used - a twin triode tube featuring low inner resistance, medium degree of amplification and large plate-dissipation, which is exclusively developed for high-voltage-driver stage. Therefore, the 6240G is quite suitable for the driver tube of giant output power amplifier. We put a reasonable difference on the plate-resistors to obtain equivalent gain between the two driver tubes."

 

For the second stage, I've bought 3 x NOS Sylvania 6GU7 tubes, which have almost identical characteristics to the 12BH7 tube that XLR8R recommended, but doesn't need an adapter:

 

Measure

Description

Units

 6240G

6GU7

12BH7

Vf

Heater voltage

V

6.3

6.3

6.3

Heater current

mA

600

600

600

μ

Amplification factor

mu

35

17

16.5

Ia

Plate current

mA

7

11.5

11.5

S

Mutual conductance

mA/V

 

3.1

3.1

Pa

Plate dissipation

W

3

3

3.5

Va max

Max plate voltage

V

800

330

300

Vp

Plate voltage

V

250

250

250

Vg

Grid voltage

V

-5

-10.5

-10.5

Gm/Rg

Transconductance

μmhos

3500

3100

3100

Rt/Ri

Plate resistance

ohms

 

5500

5300

 

So, having bought three tubes because the seller had three left and it seemed like a good idea to have a spare, do I just switch the 6240Gs out for a random pair of them and try them out? Do these driver tubes do a side each, which means they should be a matched pair?

 

 

 

Edited by Luckiestmanalive
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The first tube is shared one half for each channel so that one needed matched halves, but the second tubes are each for each channel.

 

As for what to do, try them and if it sounds unbalanced play musical tubes with the three tubes :)

 

Edit: with small signal; tubes that are NOS I generally don't worry about matching, but if each half is used in separate channels I tend to go for matching of halves.

Edited by muon*
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The most important parameters to match when substituting tubes for other types is to look for like for like characteristics for plate and grid voltages. As suggested before, the E80CC with tube adapter would be the closest option to the 6240G. The 6CG7 would be the next closest option to the 6240G using a tube adapter.

Edited by xlr8or
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3 hours ago, Luckiestmanalive said:

Ah, yes, I see - OK. You have tried the 12BH7, though, right?

 

I've tried the 12BH7 and many other tube types. My recommendation above for using the E80CC is based on many years of experience in rolling many and I mean many tube types in the A3600, and the many other tube amps I own. My advice is stick with the 6240G until you acquire an E80CC pair with tube adapters. Passing on free advice that would cost many here an arm and a leg to buy and tube roll nowadays.

Edited by xlr8or
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Thanks, K. What's annoying is I think I meant to push the button on the other Miniwatt. Never mind, I will try a couple of others I can find relatively cheaply.

 

On the question of the suitability of the 6GU7, I found reference to it being a preferred replacement for the 6240G in the Luxman MB3045 here:
https://audiokarma.org/forums/index.php?threads/i-finally-got-a-pair-of-luxman-mb-3045-mono-blocks-of-my-own.458770/

 

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Yikes - it appears that E80CC tubes are hard to get and relatively expensive so, while I think about it and look for a good deal, I've done some tube rolling with what I have.

 

I've tried the NOS 6GU7 tubes and I think they are a slight improvement on the original 6240Gs, but it isn't so obvious so I could be just imagining it. The 6GU7s are from this fleabay auction:
https://www.ebay.com.au/itm/184841552221

 

Then I swapped the ECC85 tu1 Miniwatt (77/72) for a Zaerix 6aQ8/ECC85 (11.6/11.8) that was pretty cheap:

https://www.ebay.com.au/itm/284531082399

 

It seems to have pushed the musicians back a little but the imaging appears to be a little less lop-sided.

 

It appears in my ebay flurry (it was late at night and I'd been listening and might have also had a beer and whisky or two) I also bought a matched pair of NOS 6N1P-EV tubes (they haven't arrived yet):

https://www.ebay.com.au/itm/393820455549

 

However, looking at my table and comparing the characteristics of the 6240G and 6N1P-EV, they seem to be quite a bit different:

 

Measure

Description

Units

 6240G

6GU7

E80CC

6N1P-EV

Vf

Heater voltage

V

6.3

6.3

6.3

6.3

Heater current

mA

600

600

600

600

μ

Amplification factor

mu

35

17

27

35

Ia

Plate current

mA

7

11.5

6

7.5

S

Mutual conductance

mA/V

 

3.1

2.7

4.5

Pa

Anode/Plate dissipation

W

3

3

2

2.2

Va max

Max plate voltage

V

800

330

300

300

Vp

Anode/Plate voltage

V

250

250

250

250

Vg

Grid voltage

V

-5

-10.5

-5.5

-1.5

Gm/Rg

Transconductance

μmhos

3500

3100

5000

7500

Rt/Ri

Plate resistance

ohms

 

5500

10000

4400

 

In the main, the grid voltage and transconductance...

 

Anyway, this whole thing (tube amp, biasing, tube rolling) has really boosted my enthusiasm for listening to music. Every track has something exciting to listen to I look forward each day to turning it on (waiting) and playing different tracks and full albums.

 

Even with the tubes I have now, I can spend some time trying different combinations. However, the next and harder decision is about replacing the output tubes but I will start another thread for them...

Edited by Luckiestmanalive
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  • 1 year later...
On 24/07/2022 at 2:42 PM, Luckiestmanalive said:

Yikes - it appears that E80CC tubes are hard to get and relatively expensive so, while I think about it and look for a good deal, I've done some tube rolling with what I have.

 

I've tried the NOS 6GU7 tubes and I think they are a slight improvement on the original 6240Gs, but it isn't so obvious so I could be just imagining it. The 6GU7s are from this fleabay auction:
https://www.ebay.com.au/itm/184841552221

 

Then I swapped the ECC85 tu1 Miniwatt (77/72) for a Zaerix 6aQ8/ECC85 (11.6/11.8) that was pretty cheap:

https://www.ebay.com.au/itm/284531082399

 

It seems to have pushed the musicians back a little but the imaging appears to be a little less lop-sided.

 

It appears in my ebay flurry (it was late at night and I'd been listening and might have also had a beer and whisky or two) I also bought a matched pair of NOS 6N1P-EV tubes (they haven't arrived yet):

https://www.ebay.com.au/itm/393820455549

 

However, looking at my table and comparing the characteristics of the 6240G and 6N1P-EV, they seem to be quite a bit different:

 

Measure

Description

Units

 6240G

6GU7

E80CC

6N1P-EV

Vf

Heater voltage

V

6.3

6.3

6.3

6.3

Heater current

mA

600

600

600

600

μ

Amplification factor

mu

35

17

27

35

Ia

Plate current

mA

7

11.5

6

7.5

S

Mutual conductance

mA/V

 

3.1

2.7

4.5

Pa

Anode/Plate dissipation

W

3

3

2

2.2

Va max

Max plate voltage

V

800

330

300

300

Vp

Anode/Plate voltage

V

250

250

250

250

Vg

Grid voltage

V

-5

-10.5

-5.5

-1.5

Gm/Rg

Transconductance

μmhos

3500

3100

5000

7500

Rt/Ri

Plate resistance

ohms

 

5500

10000

4400

 

In the main, the grid voltage and transconductance...

 

Anyway, this whole thing (tube amp, biasing, tube rolling) has really boosted my enthusiasm for listening to music. Every track has something exciting to listen to I look forward each day to turning it on (waiting) and playing different tracks and full albums.

 

Even with the tubes I have now, I can spend some time trying different combinations. However, the next and harder decision is about replacing the output tubes but I will start another thread for them...

Hi Greg, Can the 6GU7 replace the 6240 in my monos ?  Safe?  

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On 29/10/2023 at 12:48 PM, couchy said:

Hi Greg, Can the 6GU7 replace the 6240 in my monos ?  Safe?  

 

I've attached an image taken from another website indicating possible subs. I'd recommend using the 6CG7. However, if you want 6240G tubes then they can be found on Yahoo Aleado.

 

Screenshot_20231030-133856_Drive.thumb.jpg.83d01c624c5e7f3ecc6367c34817bda4.jpg

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Hi Craig,

 

I have a dozen 6CG7/6FQ7s from Hitachi, NEC and GE. I also have a dozen 6GU7s from Hitachi and GE. From each dozen, I have one pair of each tube type that appear to be closely matched wrt measured voltages (I've made a spreadsheet and inserted the voltages measured at test points with various combinations).

 

I bought 6GU7s because I read a forum post on this page from someone who was using them in his MB3045s (the recommendation was for Sylvania short-plate 6GU7s, in particular). I can't tell you if they will work as well or better on your MB3045s. I can just tell you that I'm preferring 6GU7s to 6CG7s in my A3600 amplifier and they seem to be working very happily, according to the voltages and temperatures I've taken and the calculations I've made on vtadiy.com.

Edited by Luckiestmanalive
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Hi Greg, I read that somewhere too about the 6gu7's.  I think the 6cg7 was used in the luxman  d-105u cd player.  I did a quick comparison between  the mb3045b   luxman m-7f and  the  luxman 38fd mkII  and the  monos  sounded best to me. They all sound great. I also have the M--4000u to compare to one day. I just bought  another pair of the 8045g from Japan. Hope they get here in one piece. I could try and get the 6240g spares from the same source. Happy listening.

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5 hours ago, xlr8or said:

 

I've attached an image taken from another website indicating possible subs. I'd recommend using the 6CG7. However, if you want 6240G tubes then they can be found on Yahoo Aleado.

 

Screenshot_20231030-133856_Drive.thumb.jpg.83d01c624c5e7f3ecc6367c34817bda4.jpg

Thanks, I have just ordered a pair  of 8045g  from Japan and I might try to get the 6240g spares from there too.

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Yes, getting good originals would best preserve the Tim Paravicini's design. I haven't tried 8045Gs because I'd need to undo the circuit modifications on mine but I did buy a couple of used 6240Gs. Unfortunately, one side of one of them was bad and one power tube wouldn't hold its bias so that was a waste of money. I did find a European website that sold new copies of 6240Gs but I can't seem to find it now.

Edited by Luckiestmanalive
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Interesting about the 6240g new copies. From what I've read in the past the 8045g sounds better than almost all modified units.  Next I will get out my Cl-35 mark 3  and pair with the monos. That also  is in near mint condition.

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  • 2 weeks later...
On 31/10/2023 at 5:48 AM, Luckiestmanalive said:

Yes, getting good originals would best preserve the Tim Paravicini's design. I haven't tried 8045Gs because I'd need to undo the circuit modifications on mine but I did buy a couple of used 6240Gs. Unfortunately, one side of one of them was bad and one power tube wouldn't hold its bias so that was a waste of money. I did find a European website that sold new copies of 6240Gs but I can't seem to find it now.

 Hi Greg , my 8045g pair arrived from Japan ,briefly tested in amp and they work so now I have 4 spares. Listening to these monos right now and they never cease to amaze me with their sound. I have not ordered any 6240g yet, wanted to see how the other tubes worked out first. I want to change those caps but i am also wondering if I should leave well alone while they sound so good.

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  • 1 month later...

One of the important differences I've found while swapping input and driver tubes in and out of my A3600 is that the DC coupling in the first two stages (ie no interstage decoupling cap) seems to make matching/balancing more important. Material differences in the plate voltages on the two sides of the 6AQ8 input tube are passed to the grids of the two phase splitter tubes, adding to differences inherent in each side of the two tubes.

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