Exactly 7 weeks since I started the metal work I have powered up the amp!
The good news is it didn't explode or start the house on fire.
( I get a little nerves every time I fire up something with450VDC!)
BUT, I had a few of screw-ups.
1. I forgot the 1M resistors across the input to ground.
(I caught that by checking the resistance checks as explained in the 8B service manual.)
2. Had to tweak the bias supply to allow for a wider range.
3. The B+ is a little too high, right now I have 470V at the plates of the EL34's,
so I'm running them at 45mA for the moment.
4. And the main screw up.... I had the opt's wired wrong, so when I add the feedback resistor to the circuit, I increased the input sensitivity instead of lowering it.... another words I added positive feedback.
BTW, THIS IS NOT A FIRST TIME BUILDERS PROJECT
THIS IS STILL A WORK IN PROGRESS
I will say I love how this amp turned out cosmetically!
All I've done so far is some scope testing with a resistor load.
More to come...
The good news is it didn't explode or start the house on fire.
( I get a little nerves every time I fire up something with450VDC!)
BUT, I had a few of screw-ups.
1. I forgot the 1M resistors across the input to ground.
(I caught that by checking the resistance checks as explained in the 8B service manual.)
2. Had to tweak the bias supply to allow for a wider range.
3. The B+ is a little too high, right now I have 470V at the plates of the EL34's,
so I'm running them at 45mA for the moment.
4. And the main screw up.... I had the opt's wired wrong, so when I add the feedback resistor to the circuit, I increased the input sensitivity instead of lowering it.... another words I added positive feedback.
BTW, THIS IS NOT A FIRST TIME BUILDERS PROJECT
THIS IS STILL A WORK IN PROGRESS
I will say I love how this amp turned out cosmetically!
All I've done so far is some scope testing with a resistor load.
More to come...
Attachments
Forgetting the resistors to ground? OPT wired incorrectly? The whole thing with the water decals being black instead of white?
Well, seeing the fact that this amplifier build of yours is so screwed up, I strongly suggest that you start a new 8b build right from zero. This is the only way that you will feel right about it all.
I will take on the grim task of disposing of this current abomination of sonic amplification in an environmentally safe manner for the low price of $20. Now, I know what you are saying, "overtheairbraodcast is a generous guy... others would charge me waaaaaaay more."
Just make sure you pack it well and send it to me with insurance...
No, it looks great and tell me of any diyaudio build that has gone perfectly the first time and I got some land just off the coast of Florida I can sell you.
I can not wait to see the amplifier in it's native habitat set up and glowing. Thanks for the pictures so far. We need more.
Well, seeing the fact that this amplifier build of yours is so screwed up, I strongly suggest that you start a new 8b build right from zero. This is the only way that you will feel right about it all.
I will take on the grim task of disposing of this current abomination of sonic amplification in an environmentally safe manner for the low price of $20. Now, I know what you are saying, "overtheairbraodcast is a generous guy... others would charge me waaaaaaay more."
Just make sure you pack it well and send it to me with insurance...
No, it looks great and tell me of any diyaudio build that has gone perfectly the first time and I got some land just off the coast of Florida I can sell you.
I can not wait to see the amplifier in it's native habitat set up and glowing. Thanks for the pictures so far. We need more.
Overtheairbroadscast.... Sorry, $20 is a bit low now that I fixed the OPT wiring issues
I had the plates backwards, for the LL1663, Plate 1 (pin#5) goes to the "top" EL34
So after swapping a couple of wires on the OPT's things are fine.
I'm using a 1.2K resistor from the 8R OPT tap for -8dB feedback
.52Vac in = 9.5Vac at the grid of the EL34's
With the EL34's at 468V@45mA I measured 28W before clipping, in TRIODE mode!
SY, YES I contact you latter for some measurements
I had the plates backwards, for the LL1663, Plate 1 (pin#5) goes to the "top" EL34
So after swapping a couple of wires on the OPT's things are fine.
I'm using a 1.2K resistor from the 8R OPT tap for -8dB feedback
.52Vac in = 9.5Vac at the grid of the EL34's
With the EL34's at 468V@45mA I measured 28W before clipping, in TRIODE mode!
SY, YES I contact you latter for some measurements
Here's an update:
I've been listening to the amp for 4 nights now, lets say 8-10 hours.
I know that there is a break-in period required so it's too early to comment on how it sounds, But for now here is my first impression:
My first thought was that it's totally different than others I have built. This is the first time I have built an amp using all carbon comp resistors and vintage NOS coupling caps. So maybe that's what I'm hearing.
In a month or so I will do an A/B test of the 8B replica compared to my 300B SE.
According to the measured specs, the 8B replica beats my 300B hands down.
But long time ago on a thread I had for a WE91 build, someone on this forum asked "do you listen to your scope"
At first I thought the guy was being a wise a**, but there may be a lot of truth to his comment. Stay tuned for the 300B comparison.
For the specs on the 8B replica, here is what I have so far:
10Hz -20K Hz with less than .5dB distortion
68K Hz at +3dB
(measured at 1 watt)
These are the 2nd best specs of the 8 amps I have built. Only there KT88 amp with Plitron opt's (which cost about $400 each) had better specs.
Next on the list is to tweak the power supply to get closer to the 435V B+ as the original....
Also need on install some sound deadening material (cork) inside the transformer cover. I have some transformer buzz.
More to come....
I've been listening to the amp for 4 nights now, lets say 8-10 hours.
I know that there is a break-in period required so it's too early to comment on how it sounds, But for now here is my first impression:
My first thought was that it's totally different than others I have built. This is the first time I have built an amp using all carbon comp resistors and vintage NOS coupling caps. So maybe that's what I'm hearing.
In a month or so I will do an A/B test of the 8B replica compared to my 300B SE.
According to the measured specs, the 8B replica beats my 300B hands down.
But long time ago on a thread I had for a WE91 build, someone on this forum asked "do you listen to your scope"
At first I thought the guy was being a wise a**, but there may be a lot of truth to his comment. Stay tuned for the 300B comparison.
For the specs on the 8B replica, here is what I have so far:
10Hz -20K Hz with less than .5dB distortion
68K Hz at +3dB
(measured at 1 watt)
These are the 2nd best specs of the 8 amps I have built. Only there KT88 amp with Plitron opt's (which cost about $400 each) had better specs.
Next on the list is to tweak the power supply to get closer to the 435V B+ as the original....
Also need on install some sound deadening material (cork) inside the transformer cover. I have some transformer buzz.
More to come....
Attachments
The Speakers are DIY Model 19 with a modified crossover....
(Replaced the L-Pads with an Autoformer and 3 position switch for -3dB,-6dB -9dB)
Here a link to the Speaker Build... (But not the modified crossover)
Model 19 Build Questions - AudioKarma.org Home Audio Stereo Discussion Forums
(Replaced the L-Pads with an Autoformer and 3 position switch for -3dB,-6dB -9dB)
Here a link to the Speaker Build... (But not the modified crossover)
Model 19 Build Questions - AudioKarma.org Home Audio Stereo Discussion Forums
SY, Yes, I would like to take up on this offer. I sent you a private message.
Tonight I modified the power supply to get a lower B+
The "final" power supply schematic is attached
Here what I ended up with
B+ is 450V
PI 425V
Input 407V
I have 445V at the plates of the EL34's....
(445V, -38.5V, 60mA)
That's only a 5V drop across the OPT's! Talk about a low "insertion" loss. That works out to .1dB
Even with the lower B+ I'm still getting 24W of power (14vac across 8.2R) before clipping.
According to the Mullard EL34 data sheet, the output power is 19W with the EL34's running at 400V/70mA (in triode mode)
Attachments
Here's the "Final" schematic for the amp and the bias supply.
I'll say "Final" for now because I'm still experimenting with the cross-coupled caps
that go from the EL34 plates to the grids of the PI
I measure better frequency response without them, so for now, the 1.5pF caps are not connected
I believe that all the info you need to build this amp is now posted in this thread.
Some notes:
My amp is based on PA-060 Power Transformer (Triode Electronics)
and Lundhal Choke and OPT's wired in Triode mode.
Other than my running the EL34's in triode mode, the replica amp is about as close as you can get to the original circuit using standard (off the shelf) iron.
I'll say "Final" for now because I'm still experimenting with the cross-coupled caps
that go from the EL34 plates to the grids of the PI
I measure better frequency response without them, so for now, the 1.5pF caps are not connected
I believe that all the info you need to build this amp is now posted in this thread.
Some notes:
My amp is based on PA-060 Power Transformer (Triode Electronics)
and Lundhal Choke and OPT's wired in Triode mode.
Other than my running the EL34's in triode mode, the replica amp is about as close as you can get to the original circuit using standard (off the shelf) iron.
Based on SY's measurements and recommendations I tweaked the feedback circuit.
With the original 1.2K feedback resistor I measured +2.9dB at 70K Hz
With the new 680pf//1.2K, +2.6dB at 100K Hz
Not bad for a 50 cents capacitor!
And speaking of capacitors... I have decided NOT to use the 2 cross coupled capacitors. They didn't make any improvement for my amp.
My measurements where made the old fashion way with a scope and single generator. After my visit with SY, a Spectrum Analysis program is next on the wish list!
With the original 1.2K feedback resistor I measured +2.9dB at 70K Hz
With the new 680pf//1.2K, +2.6dB at 100K Hz
Not bad for a 50 cents capacitor!
And speaking of capacitors... I have decided NOT to use the 2 cross coupled capacitors. They didn't make any improvement for my amp.
My measurements where made the old fashion way with a scope and single generator. After my visit with SY, a Spectrum Analysis program is next on the wish list!
Attachments
That's a good question and most likely it will be for sale within 12 months.
I'm all ready planning the next amp.
I would hope I could get $2000 min, even as high as $2500.
If you look at the originals that sell on ebay, the rough ones sell for at least $2200, and the good ones are at least $3500
I would like to know what you folks think the amp is worth.
Also, for the next project I'm thinking about a tube rectifier version of the 8B
My question is:
What would have the best re-sale value
A. A tube rectified 8B (which would no longer be even close to a "Replica"
B. Marantz Model 5 "Replica" (mono blocks)
The power transformer of the current amp is perfect for a dual rectifier (GZ34) version on an 8B.... I make the amp about 2" wider and put the capacitors under the transformer cover. The two new rectifier tubes would be above the meter and bias test switch.
Don't do it! The Marantz 8B, McIntosh 2nn models, and the H/K Cit. 2 are revered, in no small part, due to the high current, low impedance, SS rectified, B+ PSUs employed. The superior bass performance delivered is directly related to SS rectified B+.
JMO, look at "noiseless" high PIV Schottky diodes, for B+ rectification and also search the archives for my posts about "hash" filtration.
If you are absolutely determined to use vacuum rectification, don't use the relatively wimpy 5AR4/GZ34. Instead, use 6CJ3 damper diodes.
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