Prototype 125 watt monoblock tube amplifier KIT

by **Bob Latino** Thu Sep 09, 2010 5:43 pm

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Post n°26

Re: Prototype 125 watt monoblock tube amplifier KIT

by Bob Latino Thu Sep 09, 2010 5:43 pm

baddog1946 wrote:Hi Bob:
quick question, is the main transformer the same as the 120 or is it a custom built as well?
Baddog1946

Baddog,

The power and output transformers are both custom production transformers. The specs could change on both transformer as the prototype could evolve into a production item. At this time I don't want to give out any more info on the transformers used in the M-125.

Bob

by **Bob Latino** Thu Sep 09, 2010 6:59 pm

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Post n°27

Re: Prototype 125 watt monoblock tube amplifier KIT

by Bob Latino Thu Sep 09, 2010 6:59 pm

Hi Peter,

To get the most out of this amp you should use the Weber WZ68 Copper Cap solid state rectifier which can flow 450 milliamps if necessary. The WZ68 is equal to two GZ34 tube rectifiers. If you use ONE Weber WZ68 in this amp (or the ST-120) you HAVE (in effect) TWO GZ34 tube rectifiers. This M-125 amp may also be used with a single GZ34 tube rectifier at a slightly reduced power output.

Bob

by baddog1946 Thu Sep 16, 2010 6:39 pm

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Post n°28

Re: Prototype 125 watt monoblock tube amplifier KIT

by baddog1946 Thu Sep 16, 2010 6:39 pm

Hi Bob:
I was looking at your prototype and I noticed that the switch on top which I assume is the triode/ultralinear switch is a single. Is there only one switch for all four tubes? How does that work?

by **Bob Latino** Thu Sep 16, 2010 6:55 pm

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Post n°29

Re: Prototype 125 watt monoblock tube amplifier KIT

by Bob Latino Thu Sep 16, 2010 6:55 pm

Hi Baddog,

Yes - there is only one DPDT triode/ultralinear switch. The DPDT switch goes to a terminal strip which has two lugs. One lug is for the left two tubes and one lug is for the right two tubes.

When the switch faces the power transformer it connects the two tubes on each side to the ultralinear screen tap. The GREEN/WHITE ultralinear wire goes to the left two tubes. The GREEN wire goes to the right two tubes. The amp is now set for pentode/ultralinear operation.

When the switch faces the front of the amp it disconnects both ultralinear lines and then connects four resistors coming off the terminal strip (one for each the 4 output tubes) which tie the screen tap and plate together on each tube. I don't want to give the value of these 4 resistors right now. I am right now in the process of experimenting with different resistor values and checking the sound quality and proper operation of the amp.

Bob

by davidness Fri Sep 17, 2010 12:03 pm

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Post n°30

Size

by davidness Fri Sep 17, 2010 12:03 pm

Bob Latino wrote:This amp is 10 X 12 inches (120 square inches).

So, a pair will not sit side-by-side in a standard IT rack opening; 17.5"? Oh well.

BTW, I'll want to be in the first group that you start selling this kit to, as long as the price is reasonable.

A cool upgrade?... Take some of the final chassis and have them gold anodized before screen printing. With matching transformer housings, this would be just what I'd be looking for.

How do you think these will stack up against the McIntosh MC2100 or MC2102?

by avi.inc Fri Sep 17, 2010 12:25 pm

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Post n°31

Rack space

by avi.inc Fri Sep 17, 2010 12:25 pm

davidness wrote:

Bob Latino wrote:This amp is 10 X 12 inches (120 square inches).

So, a pair will not sit side-by-side in a standard IT rack opening; 17.5"? Oh well.

BTW, I'll want to be in the first group that you start selling this kit to, as long as the price is reasonable.

A cool upgrade?... Take some of the final chassis and have them gold anodized before screen printing. With matching transformer housings, this would be just what I'd be looking for.

How do you think these will stack up against the McIntosh MC2100 or MC2102?

A pro-rack is 19" between the ralls and just over 20" behind them.So maybe with the right shelf and load from the rear.

by **Bob Latino** Mon Sep 27, 2010 4:23 pm

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Post n°32

Re: Prototype 125 watt monoblock tube amplifier KIT

by Bob Latino Mon Sep 27, 2010 4:23 pm

Prototype 125 watt monoblock tube amplifier KIT - Page 2 M-125schematic_2_NV

The circuit for the M-125 as shown above is pretty well worked out now although there could be some changes. If there are any major changes I will pull the photo above and replace it with any updated circuit. The driver circuit is a modified VTA Mark III circuit using two 12BH7 tubes for higher voltage and current swings. The output is a classic parallel push pull circuit.

An interesting sidelight is that this amp may be used with FOUR KT88 (or 6550, KT90, KT120) output tubes for about 125 watt output OR it may ALSO be used with just TWO of the above tubes for about 65 - 70 watts. If used with just two output tubes, you must use one left side and one right side tube. It doesn't matter which ONE of the two left side tube sockets is used or which ONE of the two right side tube sockets is used. The amp must be rebiased when switching to two tube operation. I don't want to give out the bias points just yet because I am still experimenting with different bias settings.

Later this week I will be making up the second M-125. I will keep you posted on further progress later this week or early next week ...

I have updated the slight circuit changes on 10/11/10.

Bob

Last edited by Bob Latino on Tue Nov 09, 2010 5:56 pm; edited 4 times in total

by **Bob Latino** Sat Oct 02, 2010 4:48 pm

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Post n°33

Re: Prototype 125 watt monoblock tube amplifier KIT

by Bob Latino Sat Oct 02, 2010 4:48 pm

Prototype 125 watt monoblock tube amplifier KIT - Page 2 M-125pair1280

I built the second prototype M-125 amp yesterday and it also works fine. I altered the circuit photo in the above post because I made some changes. The inside wiring on both amps does not look neat since I have moved wires around about 10 times. I won't have a chance to listen to both critically as a stereo pair until sometime next week. I will let you know how things go by the middle of next week. I am using the same tube setup in both amps. A Weber WZ68 SS rectifier, four Sovtek KT88's and two ElectroHarmonix 12BH7 driver tubes.

Bob

by DesignInc Mon Oct 04, 2010 1:09 pm

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Post n°34

Re: Prototype 125 watt monoblock tube amplifier KIT

by DesignInc Mon Oct 04, 2010 1:09 pm

WoooooHooooooooo Dang Bob, they look awesome

David

by **Bob Latino** Tue Oct 05, 2010 1:34 pm

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Post n°35

Re: Prototype 125 watt monoblock tube amplifier KIT

by Bob Latino Tue Oct 05, 2010 1:34 pm

Two VTA M-125's cranking out the tunes ...

Well the wife went to visit her mother early this morning today so I turned the amps on at 8:30 this morning and gave these two amps a real workout with a variety of different types of music for about 5 hours. I used the amps about equally in pentode ultralinear and triode mode. I can tell that I took the photo above with the amp in triode mode because the two switches are facing the driver board. No blown fuses, funny noises etc. for the 5 hours. These M-125's are even quieter than either the VTA ST-70 or ST-120's. And those two amps are pretty quiet to begin with. One of the reason's is probably the slightly lower gain level of this amp. Another reason could be that there is no interaction between two channels. Attention was paid to wire routing schemes.

The amps give a large soundstage with good depth and and excellent WIDTH. This extra width is usually characteristic of most monoblocks anyhow where there is no interaction with another channel on the amp. At times, on certain recordings, I could hear sound outside the width of the speakers. IOW - a little to the left of the left speaker and a little to the right of the right speaker. There is an impression of "more mid bass and deep bass" on these amps over the ST-120 but this is just a characteristic of having 3 to 4 dB of extra headroom on orchestral peaks. I had never listened so loudly in the triode mode before. This amp sounds much like the VTA ST-120 at lower volume levels. The circuit is similar to one channel of the ST-120 but the resistor set is completely different and the driver tubes are 12BH7's which can give larger voltage swings than most 12**7 driver tubes. In the photo I am using two of the BIG(!) Mullard GZ33 rectifiers in place of the Weber WZ68 just to try something different. Not much difference in the sound of the amps between the Mullard GZ33 and the Weber WZ68. A GZ33 is rated at 300 milliamp current flow with a choke input. If you play these amps loud a lot, it is probably best use the Weber WZ68 in these amps. It can flow up to 450 milliamps when necessary. I am also biasing these output tubes a little higher (60 milliamps per tube) which is right about at the limit of a GZ34 tube rectifier.

I probably won't be able to listen critically again until next week sometime. I will play with the two output tube option a little next time. As I mentioned above, you can also use these amps with just TWO output tubes instead of four for about 65 to 70 watts per monoblock.

Bob

Last edited by Bob Latino on Sat Nov 06, 2010 6:49 pm; edited 1 time in total

by baddog1946 Tue Oct 05, 2010 3:53 pm

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Post n°36

Re: Prototype 125 watt monoblock tube amplifier KIT

by baddog1946 Tue Oct 05, 2010 3:53 pm

Hi Bob:
Well those amps certainly look impressive. What do you calculate the gain at? Did you use wire hookup or direct component connections? What preamp are you running them into? I noticed you haven't got around to the stepped attenuators on them would they work suitably or are you going to stay with a preamp? Any thoughts on the kits and how much? Very tidy looking package congratulations! Sorry to be so inquisitive but I can't help myself. I may be one of your first customers if I don't have to mortgage my shack in the jungle.
Baddog1946

by **Bob Latino** Tue Oct 05, 2010 4:20 pm

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Post n°37

Re: Prototype 125 watt monoblock tube amplifier KIT

by Bob Latino Tue Oct 05, 2010 4:20 pm

baddog1946 wrote:Hi Bob:
Well those amps certainly look impressive. What do you calculate the gain at? Did you use wire hookup or direct component connections? What preamp are you running them into? I noticed you haven't got around to the stepped attenuators on them would they work suitably or are you going to stay with a preamp? Any thoughts on the kits and how much? Very tidy looking package congratulations! Sorry to be so inquisitive but I can't help myself. I may be one of your first customers if I don't have to mortgage my shack in the jungle.
Baddog1946

Hi Baddog,

The gain is about 1 or 1.1 volts in for 125 watts out - slightly less sensitive than the ST-120. I am running a BAT (Balanced Audio Technology) VK-3i preamp into the M-125's. There won't be any stepped attenuator option on the M-125 - you will have to use a preamp. The attenuators that I have are stereo attenuators and are for the stereo ST-70 or ST-120 amps.

I have done a little cost analysis on the M-125 and a SINGLE M-125 will cost a little less than an ST-120. Maybe about $1700 - $1750 for a PAIR of M-125 KITS. That doesn't include tubes. The M-125's will come with both cap upgrades as standard equipment. I won't be stocking the tube sets for the M-125 like I do on the ST-70 and ST-120. I just don't have the $$$ to stock the tube sets for this amp also. Another thing is that I won't be offering these as wired amps at least not at first. (NOTE - The M-125's are now also available as wired amps - See my web link below for more details). It would take me a week's work to wire up two of these amps and in that time I couldn't pack up any kits for kit customers.

VTA tube amp kits

Bob

Last edited by Bob Latino on Thu Dec 03, 2015 1:14 pm; edited 2 times in total

by **Bob Latino** Thu Oct 07, 2010 7:50 pm

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Post n°38

Re: Prototype 125 watt monoblock tube amplifier KIT

by Bob Latino Thu Oct 07, 2010 7:50 pm

Prototype 125 watt monoblock tube amplifier KIT - Page 2 RunningTWOoutputtubes_1024

I ran the M-125 amps all day today in the "two output tube mode". The two output tubes are the newer Tung-Sol KT120 tubes. These tubes sound just a little more "tubier" than KT88's - a little smoother. Almost like running the amp in triode but with more power. On Wednesday next week I will pick up two (what should be) PRODUCTION chassis with (hopefully) all the holes in exactly the right spots. I will have to mount all the parts to double check that everything is OK. If it is OK - I will post a photo sometime next week of the production chassis with all the parts mounted.

Bob

by baddog1946 Fri Oct 08, 2010 9:13 am

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Post n°39

Re: Prototype 125 watt monoblock tube amplifier KIT

by baddog1946 Fri Oct 08, 2010 9:13 am

Hi Bob:
When running in the two tubes per mono block mode what bias are you using for the 120's? Does this amp have higher voltages than the ST-120?
What do you estimate the output with two 120's to be.
Baddog1946

by **Bob Latino** Fri Oct 08, 2010 9:37 am

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Post n°40

Re: Prototype 125 watt monoblock tube amplifier KIT

by Bob Latino Fri Oct 08, 2010 9:37 am

baddog1946 wrote:Hi Bob:
When running in the two tubes per mono block mode what bias are you using for the 120's? Does this amp have higher voltages than the ST-120?
What do you estimate the output with two 120's to be.
Baddog1946

Hi Baddog,

I am running the KT-120's at .600 VDC each. I have run them up a little higher in this amp without much change in sound. This amp has voltages about the same as the ST-120. The KT120's may be run in this amp but the amp is optimized for KT88 and 6550 tubes. In order to get the most out of the KT120 tubes you have to get the voltages up higher (maybe to 600 volts) than this amp can provide. You also need to run them at 100 to maybe 120 milliamps each tube instead of the 60 milliamps I am using. It is on the same idea as running KT88's in an ST-70. You can do it but you won't get all that the tubes can deliver ...

NOTE - 2/17/17 - All production VTA M-125's should run .50 VDC bias on both sides using TWO output tubes and should run a bias of 1.00 VDC per each side in the FOUR output tube mode.

Bob

Last edited by Bob Latino on Fri Feb 17, 2017 9:41 am; edited 2 times in total

by baddog1946 Fri Oct 08, 2010 3:24 pm

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Post n°41

Re: Prototype 125 watt monoblock tube amplifier KIT

by baddog1946 Fri Oct 08, 2010 3:24 pm

hi Bob:
Would the WZ68 Copper cap rectifier handle that much bias with all four tubes KT-120's? Would the 12BH7's have any problem with that? I think I fried a Coppercap WZ68 because I was running 4 KT-120's in my ST-120 at 75 milliamps a couple of months ago. I went back to the KT-88's and lower bias currents but was wondering about the rectifier reliability factor if the 120's were used as a quad in this amp.
Baddog1946

by **Bob Latino** Fri Oct 08, 2010 6:24 pm

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Post n°42

Re: Prototype 125 watt monoblock tube amplifier KIT

by Bob Latino Fri Oct 08, 2010 6:24 pm

baddog1946 wrote:hi Bob:
Would the WZ68 Copper cap rectifier handle that much bias with all four tubes KT-120's? Would the 12BH7's have any problem with that? I think I fried a Coppercap WZ68 because I was running 4 KT-120's in my ST-120 at 75 milliamps a couple of months ago. I went back to the KT-88's and lower bias currents but was wondering about the rectifier reliability factor if the 120's were used as a quad in this amp.
Baddog1946

I have used the KT120's at 75 milliamps each with the Weber WZ68 with no issues. The Weber WZ68 will flow 450 milliamps. Four KT120's @ 75 milliamps = 300 milliamps. The three 12AT7's are maybe 10 milliamps each. Total = approximately 330 milliamps which is well under what the WZ68 can flow. The power transformer on the ST-120 is also rated at 450 milliamps. I don't know why your WZ68 blew. I have had the same one in my ST-120 for 2 1/2 years and maybe 3000+ hours with no problems.

You have a point though. It is probably wise to lower the bias level on the Tung-Sol KT120's to maybe 60 milliamps. They will last longer at a lower bias point ..

Bob

by baddog1946 Sun Oct 10, 2010 10:47 am

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Post n°43

Re: Prototype 125 watt monoblock tube amplifier KIT

by baddog1946 Sun Oct 10, 2010 10:47 am

Hi Bob:
You mention the fact that the KT-120's needed 100-120 milliamps and higher voltages to max their performance. That would appear to exceed the maximum output of a single WZ68 copper cap.

My question is: Do you know of a rectifier that will do the job with 4 KT-120's? Maybe two of them?

I was running 4 KT 120's in mt ST-120 for a while until I blew a WZ68 and during that time I also noticed and liked very much the sublte differences you refer to as a "tubier" sound. I also clearly noticed they had more punch in the bass and were roomier at the top end even at the lower voltages available with my ST-120.
I did an A-B comparison test with them and a set of "Gold Lion" KT-88's using some CD's by Victor Wooten and Marcus Miller who are bass mosters and a "Chesky" audiophile CD and it left me wishing I could use those KT-120's forever.However with only two of them running at lower output the point is moot.
I guess the other side of that is that the trannies may have some limitations as well. But it would be nice to be able to strike a happy medium where the KT-120's could be used to enjoy their unique and subtle qualities IMHO in our existing gear.
Any thoughts on that.
Baddog

by **Bob Latino** Sun Oct 10, 2010 11:20 am

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Post n°44

Re: Prototype 125 watt monoblock tube amplifier KIT

by Bob Latino Sun Oct 10, 2010 11:20 am

Hi Baddog,

120 milliamps of bias current per each of the four KT120 tubes in the M-125 (480 milliamps total) would exceed the flow rate of the Weber WZ68 (450 milliamps). 100 milliamps per 4 tubes combined with current needed for the 12BH7's would be right at the flow limit of a WZ68. Still - not a good thing to run a component in a tube amp at it's maximum design limit.

To my knowlege, at the present time, there is no amp available that can take advantage of the full value of the Tung-Sol KT120 output tubes. It is possible in the future that someone may create an amp that can use these tubes to full advantage. (600+ VDC B+ and 100 - 120 milliamps of bias current per tube). As I mentioned in an earlier post, the reason that the M-125 amp is not optimized for the Tung-Sol KT120 is that presently it is the only octal tube available that can deal with the high bias current and high voltages mentioned above. I would not want to optimize a commercial amp for ONE tube that is only available from ONE manufacturer. If Tung-Sol decides to pull the plug on this tube because it doesn't sell, then you are left with an amp without a source of output tubes. KT88 and 6550 tubes will "red plate" severely and probably self destruct with 600+ volts B+ and 100+ milliamps of bias current.

All that said, The KT120's are quite usable in the M-125 at the 500-520 VDC bias B+ voltage and 60 milliamp bias current per tube. Since the KT120's are being used at well under their designed voltage handling and bias current, they would probably last a long time in the M-125.

Bob

Last edited by Bob Latino on Sat Nov 06, 2010 6:53 pm; edited 1 time in total

by Falcon4646 Sun Oct 10, 2010 11:32 pm

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Post n°45

More Info on M-125 using just 2 KT120s?

by Falcon4646 Sun Oct 10, 2010 11:32 pm

I am certainly interested in purchasing two of the M-125s. I see you have preliminary postings for M-125s at the "new products coming soon" web page along with new phone preamps.

Per your page three posting on using two KT120s, it would be nice to hear a more indepth analysis of the sound as well as the power level that might be obtained using just two KT120s per M-125. If they are biased at 65 to 70 milliamps at the B+ voltage available can they supply 100 watts or what? Two KT120 are about $142 lying between 4 Sovtek KT88s and 4 Gold Lion KT88s but with a sound that is more tubier especially if the KT120 is impervious to overload and provides the claimed extreme reliability and long tube life.

by baddog1946 Sun Oct 10, 2010 11:53 pm

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Post n°46

Re: Prototype 125 watt monoblock tube amplifier KIT

by baddog1946 Sun Oct 10, 2010 11:53 pm

KT-120's for 89.95 a pair available at "Parts Connexion"

http://www.partsconnexion.com/

by **Bob Latino** Mon Oct 11, 2010 7:46 am

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Post n°47

Re: Prototype 125 watt monoblock tube amplifier KIT

by Bob Latino Mon Oct 11, 2010 7:46 am

Falcon4646 wrote:I am certainly interested in purchasing two of the M-125s. I see you have preliminary postings for M-125s at the "new products coming soon" web page along with new phone preamps.

Per your page three posting on using two KT120s, it would be nice to hear a more indepth analysis of the sound as well as the power level that might be obtained using just two KT120s per M-125. If they are biased at 65 to 70 milliamps at the B+ voltage available can they supply 100 watts or what? Two KT120 are about $142 lying between 4 Sovtek KT88s and 4 Gold Lion KT88s but with a sound that is more tubier especially if the KT120 is impervious to overload and provides the claimed extreme reliability and long tube life.

Hi,

Some time ago I had biased the KT120 up to 75 milliamps per tube and had initially thought that they sounded better at higher bias points. Now, after the tubes have fully broken in, IMHO they sound the same at 60 milliamps as they do at 75 milliamps per tube. My recommendation will be to bias them @ 60 milliamps each in the M-125. (All VTA M-125's now have a recommended bias of 50 milliamps per each output tube and NOT 60 milliamps) Biased that way, these KT-120 tubes *should* last a long time in the M-125 - longer than the Gold Lion's.

A PAIR of KT120's will not give 100 watts of power in the M-125. FOUR KT120's will give 125+ watts.

As the KT120's have broken in, I find that I like the sound more. There seems to be more mid-bass content and a smoother top end than the Gold Lion KT88's. Both tubes are very clean sounding. I am not going to say that I like one tube over the other. The reason is that in some music systems one tube may have its characteristics work with the other system components in a synergistic way. IOW, if your system seems a little "bright" then you may want to try the KT120's. OTOH, If you like or need a little more extension on the top end, I would try the Gold Lion's.

NOTE - I have not tried the two M-125 prototypes with four KT120's in each amp. Reason - I only had four of the KT120's here on hand. Last Friday I ordered four more of the KT-120's. By the end of this week I should have them here and will try both amps with the full complement of tubes.

Bob

Last edited by Bob Latino on Sat May 30, 2020 9:16 am; edited 2 times in total

by xlr8 Mon Oct 11, 2010 10:03 am

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Post n°48

Re: Prototype 125 watt monoblock tube amplifier KIT

by xlr8 Mon Oct 11, 2010 10:03 am

I was wondering if it would be just plain silly to optimize one of these for guitar duties? Reason I ask is because my buddy and I ran the line out of my little Fender Super Champ to a crown XLS 402 driving a cabinet loaded with 4 Celestion G12t-100's yesterday with great results, just without the overall body and tone of tubes. Just wondered...

by **Bob Latino** Mon Oct 11, 2010 10:21 am

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Post n°49

Re: Prototype 125 watt monoblock tube amplifier KIT

by Bob Latino Mon Oct 11, 2010 10:21 am

Hi,

If you just change out the RCA input jack to a guitar input jack there is no reason why one M-125 could not be used as a guitar amp. With the two or four output tube alternative combined with the triode/pentode switch, the amp would give you a few power/tone options.

I know at least two people who have built the VTA ST-70 from a kit with the primary intention of using the amp with their electric guitar.

Bob

by tubehd4life Mon Oct 11, 2010 8:31 pm

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Post n°50

Re: Prototype 125 watt monoblock tube amplifier KIT

by tubehd4life Mon Oct 11, 2010 8:31 pm

Will these amps be ready for kit purchase before the new year? PLEASE!

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