glasman wrote:
In the Fender reverb (as implemented in the old blackface amps). The mix resistor was 3M3. THis is extremely large and really has an effect on the overall tone. THis was the reason why Fender added the cap across the resistor was to gain back some highs. It increases the Miller effect in the recovery stage (think this is right, need to check Ma[/color][/color]lvino ). In the one tube arrangement, the mix is only 100K and has much less effect on the overall tone of the amp. I have checked this by placing a jumper on and off the 100K to determine its effect and it negible.
Gary
The Miller effect is only concerned with capacitance inside the bottle. Specifically, it's the capacitance between the grid and the anode (Cg-a) that is multiplied by the stage gain.
That being said, a driver consisting of a paralleled triode is going to have twice the Miller capacitance of a single triode of the same type. However, here we are comparing a paralleled 12AT7 as used by Fender with a single 12AX7. Given the higher gain of the 12AX7 and the difference in Cg-a, the single triode 12AX7 driver only comes out a little ahead.
glasman wrote:
In the Fender reverb (as implemented in the old blackface amps). The mix resistor was 3M3. THis is extremely large and really has an effect on the overall tone. THis was the reason why Fender added the cap across the resistor was to gain back some highs. It increases the Miller effect in the recovery stage (think this is right, need to check Ma[/color][/color]lvino ). In the one tube arrangement, the mix is only 100K and has much less effect on the overall tone of the amp. I have checked this by placing a jumper on and off the 100K to determine its effect and it negible.
Gary
The Miller effect is only concerned with capacitance inside the bottle. Specifically, it's the capacitance between the grid and the anode (Cg-a) that is multiplied by the stage gain.
That being said, a driver consisting of a paralleled triode is going to have twice the Miller capacitance of a single triode of the same type. However, here we are comparing a paralleled 12AT7 as used by Fender with a single 12AX7. Given the higher gain of the 12AX7 and the difference in Cg-a, the single triode 12AX7 driver only comes out a little ahead.
MPM
I think you missed what I was getting at with the 3M3. It along with the miller capacitance sets up a low pass filter for the clean signal into the reverb recovery amp. The stage gain in a Fender is around 78 on the reverb recovery. This gives a miller capacitance of about 191pf. The miller capacitance along with series resistance of 3M3 sets the break point for the high end response at well less than 1kHz. The 10pf cap helps to regain some of the top end lost due to the 3M3 clean mix resistor.
I wrote it incorrectly in my original post. The 10pf helps to decrease the miller effect on top end.
Gary
Located in the St Croix River Valley- Afton, MN
About 5 miles south of I-94
aka K0GWA, K0 Glas Werks Amplification www.glaswerks.com
No, I understand; the small cap bypasses the 3M3 and decreases the attenuation of highs by the LPF which is formed by it and the Miller capacitance.
I think we are talking semantics here- my use of the term Miller Effect is strictly in reference to the increase in Cg-a by stage gain, and nothing outside of the bottle impacts it except the stage gain as realized in-circuit. That's being a little pedantic, maybe, but accurate.
I think what you refer to when you said Miller effect is the net effect of the LPF, which includes the Miller capacitance. Is that right?
No, I understand; the small cap bypasses the 3M3 and decreases the attenuation of highs by the LPF which is formed by it and the Miller capacitance.
I think we are talking semantics here- my use of the term Miller Effect is strictly in reference to the increase in Cg-a by stage gain, and nothing outside of the bottle impacts it except the stage gain as realized in-circuit. That's being a little pedantic, maybe, but accurate.
I think what you refer to when you said Miller effect is the net effect of the LPF, which includes the Miller capacitance. Is that right?
MPM
Yes, the overall effect of the tubes Miller input capacitance working against the source impedance.
Gary
Located in the St Croix River Valley- Afton, MN
About 5 miles south of I-94
aka K0GWA, K0 Glas Werks Amplification www.glaswerks.com
We just finished the prototype for the Sig Tomo Fujita practice amp.. Single ended 6L' running a true Class A..He wanted the amp with Fender type reverb... The first prototype we did we tried the single tube 12AX and though it sounded good down low... but when you turn them up they get real boingey sounding not real clean and doesn't quite sit in the mix very well..Not enough Ass! The 2nd Prototype we used a 6BM8 driver and sounded to me more transparent cleaner and way more natural..Gary wondering if you've ever tried the 6BM8?
This is the first amp we tried it on and am thinking of moving it to our Overdrive combos Toneblenders etc.. I can't wait!!
Can anyone be so kind to put the layout available again?
I'm a electronics newbie and have a hard time reading schematics...
Thanks in advance.
Fernando
Time to learn how to read schematics if you are going to be into electronics and draw your own layouts. A circuit this simple is easy. Get a pencil with a good eraser as you will make some mistakes at first . Try it.
talbany wrote:We just finished the prototype for the Sig Tomo Fujita practice amp.. Single ended 6L' running a true Class A..He wanted the amp with Fender type reverb... The first prototype we did we tried the single tube 12AX and though it sounded good down low... but when you turn them up they get real boingey sounding not real clean and doesn't quite sit in the mix very well..Not enough Ass! The 2nd Prototype we used a 6BM8 driver and sounded to me more transparent cleaner and way more natural..Gary wondering if you've ever tried the 6BM8?
This is the first amp we tried it on and am thinking of moving it to our Overdrive combos Toneblenders etc.. I can't wait!!
Tony VVT
Can you share any circuit info / values for using the 6BM8 tube? I would like to try both tubes. Thanks
This is about as simple as a speaker driver can be built with one tube, given the limited number of audio tube types still available. The 6BM8 can also drive a Fender-type reverb driver transformer directly. This would be superior to the 12AT7 customarily used, and gives more drive power at a low cost. I started here!!.. PM me if it needs tweaking... Good Luck!!
Tony VVT
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). In the one tube arrangement, the mix is only 100K and has much less effect on the overall tone of the amp. I have checked this by placing a jumper on and off the 100K to determine its effect and it negible.
