#124 Questions

[Bombacaototal]

you can also use that resistor value to change your other plate voltages too as it affects everything down the chain.

Thanks Marcus, that is actually a point I had not realised, that everything down the chain would be impacted.

I have a couple of questions:
1) is ZA (15K dropping resistor) impacted the same way by the 150K ground on both scenarios A and B or will the effect on scenario B be more pronounced?
2) on the way I did it, is XA impacted as well?

[norburybrook]

if you put the 150k in then everything will be as it's meant to be all along the chain. I was just pointing out that you can use that last resistor on the dropping string to adjust voltages.


Al the B+ voltages are coming from the same starting point and are all connected so I imagine OHm's law would mean you could calculate voltage/impedance/current down that string.


It's a bit above my pay grade though so you'd need Martin or someone to jump in here and explain exactly.



Just keep soldering and don't worry about it too much as long as your're safe


MC

[Bombacaototal]

Al the B+ voltages are coming from the same starting point and are all connected so I imagine OHm's law would mean you could calculate voltage/impedance/current down that string.

Thanks Marcus, I am really puzzled now so will try to find how to calculate everything. What I am intigued about is if having ZA would be the same on scenario A (off the screen grids) and on scenario B.

[Bombacaototal]

One more question, I did a drawing (attached) of a 4x20 uF cap can.

How would I wire them in series for a 10uF total and how would I wire them in parallel for a 40uF total?

For example, if I connect A and B like in the attachment I am having them in series (ie 10uF) correct? How can I get them in parallel?

[sluckey]

The caps in that can cannot be wired in series, only parallel. Your pic shows A and B in parallel.

[Bombacaototal]

The caps in that can cannot be wired in series, only parallel. Your pic shows A and B in parallel.

Thanks sluckey for the reply, very helpful. That is perfect.

[pompeiisneaks]

Yup to wire in series they each need discreet grounds, instead of a shared one.

~Phil

[Bombacaototal]

I've been doing some studing about the pre-amp supply caps/dropping resistor, etc: http://www.valvewizard.co.uk/smoothing.html and http://www.valvewizard.co.uk/bias.html

The 2 questions I have are:
1) Why did HAD use the same dropping resistor on 100W (Twin PT) and 50W (Bassman PT)
I am assuming is that given both designs (50W and 100W) are using 6L6 (mu=8) the screen voltage (~ 400v) will be somewhat similar (400/8=50 negative).
But what if one used the same design (ODS #124 for example) with KT66 (mu=7.5) or 6V6 (mu=10) am I correct to assume that one would need to calculate and amend an equivalent dropping resistor to achieve the same result as the original? But then a JJ 6V6 will have ~400V on plates but given MU is different then 6L6 amendments would be required (400/10=40 negative). In the case of KT66 with a voltage of ~450V amendments would be required (450/7.5=60 negative).

Is my rationale correct?

2) What difference in sound would it make by changing V1 with 2.2K dropping resistor (ODS tonestack) from 20uF to 47uF (ie how would it impact on the cut-off frequency)
Using 20uf Cap with 2.2K resistor to calculate the RC low pass filter: R = 1 / (2 pi f C) => F = 1/ (2pi R C) = 1/ (2 pi * 2200 ohms *20×10^-6 ) = 3.62 db
Using 47uF Cap with 2.2K resistor to calculate the RC low pass filter: R = 1 / (2 pi f C) => F = 1/ (2pi R C) = 1/ (2 pi * 2200 ohms *47×10^-6 ) = 1.54 db

Which then means that 47uF will have more low end than 20uF, correct!? Is this difference audible or ignorable?

[Bombacaototal]

Yup to wire in series they each need discreet grounds, instead of a shared one.

~Phil

Thanks Phil. I am using one of the 4 random ground points of my cap can, so I think it should be alright for the parallel. It is good to know that there is the option of series as well

[erwin_ve]

1. Screen current isnt big, a few mA per tube. Bias differences can have bigger impact on voltages upstream. Using other tubes than original is gonna require a recalculation with the specs of the tube and might result in using a other transformer with different voltages/ different bias circuit. Edit: reading your post again, Im confused: are youre talking about the Psu dropping string or bias circuit?

2.bigger cap on the last node isnt just about cuttoff frequencies. It's also about feel, bouncier attack, more empasis on low end. Since english isnt my first language this is the hard to describe.
The way to experiment with this is to put a 22uF on the last node. Play with it and than tack another 22uF on top of the last one. Play with it. My findings are that it doesn't impact the tone but impacts the feel.

[Bombacaototal]

1. Screen current isnt big, a few mA per tube. Bias differences can have bigger impact on voltages upstream. Using other tubes than original is gonna require a recalculation with the specs of the tube and might result in using a other transformer with different voltages/ different bias circuit.

2.bigger cap on the last node isnt just about cuttoff frequencies. It's also about feel, bouncier attack, more empasis on low end. Since english isnt my first language this is the hard to describe.
The way to experiment with this is to put a 22uF on the last node. Play with it and than tack another 22uF on top of the last one. Play with it. My findings are that it doesn't impact the tone but impacts the feel.

1. Many thanks Erwin. I am talking about the PSU dropping string resistors. Yes, I was taking into account the different transformers in the example cases of 6V6 or KT66. In the case of my amp I am doing a 2x6V6 with Deluxe reverb PT and Vibroverb OT with 2x JJ 6V6, and unmatching the impedance of the OT (original primary of 4K) to get it up to 8K. I will try to calculate what would be the equivalent dropping resistor (JJ 6V6 data sheet attached)

435 volts on the PT plates with JJ6V6 (max is shown as 450V)
The preamp valves will consume around 6 x 1mA = 6mA
The 6V6s are rated for 14 watts maximum, so they will probably be biased around 0.7 x 14W = 9.8W.
Raw DC supply voltage is 435V then they will each consume about 14W / 435V = 32mA, or 64mA for the pair.
I am confused with data sheet. Is screen-to-anode current ratio of 4.5? If so so we can expect the screen currents to amount to 64 / 4.5 = 14mA for the pair.
The total for the whole amp is therefore 6 + 64 + 14 = 84mA.
5% ripple voltage then this would mean 2x220×0.05 = 22Vpp.
C = I / (2 f Vripple) = 0.15 / (2 × 110 × 22) = 3.1uF.

6mA × 2.2k (PI) = 13V and dissipate 0.006^2 × 2000 = 0.79 watts.

So.....what do I do next?

2. That is great feedback. I am indeed curious to experiment with this. Do you find it stiffer/bouncier with the larger cap (ie 47uF)?

[erwin_ve]

Maybe we can work the other way around this. The Ods #124, #183,#102 are well proven and documented builds. Instead of reinventing the wheel, check what is done here with a Dlite with 6V6 and then do the math: https://ampgarage.com/forum/viewtopic.p ... ilit=Dlite.

If this is your first Ods: stick with the original. If youre on the design side, like: do another kind of tube start with a smaller amp. Tweed deluxe fe.

[pompeiisneaks]

Yup to wire in series they each need discreet grounds, instead of a shared one.

~Phil

Thanks Phil. I am using one of the 4 random ground points of my cap can, so I think it should be alright for the parallel. It is good to know that there is the option of series as well

Not sure if I worded that in a way that made sense, basically I was explaining why cap cans can't be used for series. They share a common ground so you can't. They each need a discreet ground to be able to do series, and cans don't.

~Phil

[Bombacaototal]

Thanks For clarifying phil and sorry for misunderstanding your post.

Many thanks Erwin, I will look into the D’lite

For future referece a few threads about 6V6 builds:
https://ampgarage.com/forum/viewtopic.p ... plete+four
https://ampgarage.com/forum/viewtopic.p ... ur#p369665
https://ampgarage.com/forum/viewtopic.p ... plete+four
https://ampgarage.com/forum/viewtopic.p ... plete+four

[Bombacaototal]

Is it a bad idea to group the supply lead cables with the shielded ones on the same branch (as per pic attached)?

I am thinking of separating them in 2 individual ones, one for the supply leads and the other for the coax cable. If this is indeed the best approach what distance would you put them apart?