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Old 29th June 2018, 07:39 AM   #41
petertub is online now petertub  Sweden
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To increase bass : turn bass control up !

Tubes will amplify anything from DC to Mhz equally well. The circuit may affect frequency response.
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Old 29th June 2018, 07:43 AM   #42
fb2017 is offline fb2017
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Quote:
Originally Posted by petertub View Post
To increase bass : turn bass control up !
No such thing on my system, except perhaps software EQ, which I'm doing already. Many tube reviewers state they get deeper bass with such and such tube, but I realize this is very much in relationship with the amp they're using. I'll get the JJ KT77 and see what this brings .
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Old 29th June 2018, 09:32 AM   #43
DF96 is online now DF96  England
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The impression of deep bass can result from distortion, so the valves which give 'deep bass' probably need their bias adjusting to make them more linear.
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Old 29th June 2018, 09:54 AM   #44
jazbo8 is offline jazbo8
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Audion Edison 60 building and using report
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Old 29th June 2018, 04:58 PM   #45
6A3sUMMER is offline 6A3sUMMER  United States
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fb2017,

I came onto your thread late. Now I finally read through all of the posts in this thread. It is curious that nobody ever attempted to answer in regard to your scope screen shots, the ringing at the start, and the large reduction of ringing over time.

You probably do not need much power to drive those Fostex horn loaded speakers, and most of the time you are in class A (neither output tube in cut off).

I have used paraphrase splitters, and they sounded OK. But I prefer splitters that have a current source in the coupled cathodes. This almost always requires a negative supply, or has to be dealt with another way, when the input stage is the splitter (that is another subject for another thread). The cathode splitter has 6 dB less gain, and the open loop to closed loop ration is 6 dB less. Keep the splitter you have.

A good feature you have is the individual bias resistors and bypass caps for each output tube. Use matched pairs if you replace tubes, you do not have to have a matched quad. Just match the left pair; and just match the right pair.

If you want to, do put a 1 Ohm resistor for each tube, between the bottoms of the self bias resistor/bypass cap networks, and ground. You can try this for just the right or just the left channel to see the effect. Use your scope to see when you go out of class A. You might need to use 10 Ohm resistors, in order to see the signal better on your scope. You could do this when the amp is loaded with an 8 Ohm load. But when you really should do it, is when you are listening to music, at the volume you like, when the amp is connected to your loudspeakers.

If you want to try another thing, remove the output transformer screen tap wires, and insulate them. Then connect 4 100 Ohm resistors, one each from the output tube's plate to the output tube's screen. You can even just do it to one channel and compare to the other channel (2 resistors). This is triode wired mode. The maximum output power will go down, and depending on the negative feedback and loop gain, the gain may go down. But the damping factor, and the nature of the distortion will probably change.

The impression of Bass can also vary as the damping factor varies.

Continue to enjoy listening to the amp and speakers.
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Old 30th June 2018, 05:18 AM   #46
fb2017 is offline fb2017
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Hi, thank you for your very helpful post. There are a lot of very interesting ideas in here, and it gives me something to do . I haven't measured the amp in one year, I presume the ringing would be gone by now. Anyway, I am usually listening to low volumes (around 1-3W of output power), the Fostex being as efficient as they are, and I really love what I'm hearing. It is so stronlgy holographic that the speakers completely dissapear, and the soundstage move meters behind the wall. All of my friends that visited and listened to the system were quite impressed as well. I am also offering listening experiences to anyone wanting to give it a go (and wishes to travel to Germany, Lower Saxony for this ). Your 1 Ohm resistor idea, you mean between let's say C103 C104 and ground? It is unfortunately a bit difficult to implement properly, as it is not a point to point construction, but PCB. Yes, using the EL34 in triode mode was a possibility that crossed my mind. As with my speakers I'm never needing more than 10W, there is even the possibility of converting this particular amp to an 300B SET one, of about 8 Watt, that should be more than enough. Audion even added the additional filter circuit traces on the PCB for this, they sit unused in my amp. Ron Clark, the designer of the Dallas II also recommended a 1-4 Ohm resistor in the series with the speaker for enhancing the transmission line effect (by raising the speaker's Q), that would improve the bass as well.
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Old 30th June 2018, 10:38 PM   #47
6A3sUMMER is offline 6A3sUMMER  United States
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fb2017,

From your schematics in Posts #15 and #40,

Lets work on one channel at a time . . .

I expect that all the parts are mounted on the PCB.

Look at the schematic, and then at the PCB.

1. Take the negative lead of C103 out of the PCB (it is connected to ground/common) Take the end of R130 that is also connected to ground/common out of the PCB (not the end that connects to C103 positive and EL34 Cathode.

Connect the new loose ends of C103 and R130 together. Connect a 10 Ohm resistor from that new junction of C103 and R130, and the other end of the 10 Ohm resistor to the PCB where either C103 or R130 was pulled out (ground/common). That is your current sense resistor for V101. Connect scope probe ground clip to the ground/common end of the 10 Ohm resistor, and the scope probe hot tip to the other end of the 10 Ohm resistor. You can also measure the DC current of V101, with your DMM across the 10 Ohm resistor. DC Volts/10 Ohms = Amps

2. Repeat for C104 and R130, and V102. Now you can compare the current in one 10 Ohm and the other 10 Ohm, and see where you are in class A, and at what volume you are in class AB.

That is one channel.

The other channel: 3. Repeat R231 C203 V201 R230 C204 V202 Again, test DC current with a DMM, and Class A / Class AB with a scope.

Now, for Triode wiring:

Look at the schematic again.

One channel: Remove the ultra linear tap from the transformer at J106 Insulate the ultra linear tap wire Jumper J105 and J106

Remove the ultra linear tap from the transformer at J108 Insulate the ultra linear tap wire Jumper J108 and J109

(The screens already have resistors R153 and R154 on the circuit board)

The other channel: Remove the ultra linear tap from the transformer at J206 Insulate the ultra linear tap wire Jumper J205 and J206

Remove the ultra linear tap from the transformer at J208 Insulate the ultra linear tap wire Jumper J208 and J209

(The screens already have resistors R253 and R254 on the circuit board)

Triode wiring only one channel will not help you to compare, unless you have a balance control, I expect the triode wired channel might have as little as 1/2 the gain (6dB less gain), but that depends on the gain without negative feedback versus gain with negative feedback. If the open loop gain (no NFB) to closed loop gain (NFB) ratio is high enough, there will be no gain loss when triode wired.
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